Radial foil bearing and method for manufacturing radial foil bearing

Abstract

This radial foil bearing comprises: a top foil provided with an opposing surface opposing a rotating shaft; a bump foil arranged outward, that is, on the opposite side to the opposing surface, of the top foil; and a bearing housing arranged so as to surround the bump foil. The bump foil comprises: a tubular portion that surrounds the top foil in a tubular shape; and resilient piece portions which are adjacent to the tubular portion in an axial direction of the rotating shaft, are capable of deforming independently of the tubular portion, and are capable of extending in a circumferential direction. The resilient piece portions are supported by the bearing housing.

Description

[Technical Field]

[0001] The present disclosure relates to radial foil bearings and methods of manufacturing radial foil bearings. [Background technology]

[0002] For example, the radial foil bearing described in Patent Document 1 includes a thin top foil surrounding the rotating shaft, a thin bump foil surrounding the top foil, and a cylindrical housing that accommodates the top foil and the bump foil.The radial foil bearings described in Patent Documents 2 to 5 include a top foil that is elastically supported directly on the housing without including a bump foil. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-190761 [Patent Document 2] International Publication No. 2014 / 098005 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-82909 [Patent Document 4] Japanese Patent Application Publication No. 2018-150971 [Patent Document 5] Japanese Patent Application Publication No. 2019-82195 Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned radial foil bearings still have a problem in terms of providing stable support for the rotating shaft when rotating at high speeds.

[0005] The present disclosure describes a radial foil bearing and a method for manufacturing a radial foil bearing that can stably support a rotating shaft when rotating at high speeds. [Means for solving the problem]

[0006] A radial foil bearing according to one example of the present disclosure is a radial foil bearing comprising: a top foil having a facing surface facing a rotating shaft; a bump foil disposed on the outside opposite the facing surface of the top foil; and a bearing housing disposed to surround the bump foil. The bump foil comprises a cylindrical portion that cylindrically surrounds the top foil, and an elastic piece portion that is adjacent to the cylindrical portion in the axial direction of the rotating shaft, is deformable independently of the cylindrical portion, and is expandable in the circumferential direction. The elastic piece portion is supported by the bearing housing.

[0007] The bump foil has a cylindrical portion and an elastic piece portion, and the elastic piece portion is deformable independently of the cylindrical portion. The elastic piece portion is supported directly or indirectly on the bearing housing, and supports the top foil via the cylindrical portion adjacent to it in the axial direction. Furthermore, because the elastic piece portion can expand in the circumferential direction while being supported by the bearing housing, it is resistant to vibrations from a rotating shaft rotating at high speed, and can provide stable support.

[0008] In some examples, the elastic piece and the tubular portion may be integrally formed, and the bump foil may have a slit structure separating the elastic piece from the tubular portion so that the elastic piece can deform independently of the tubular portion. Because the elastic piece and the tubular portion are integrally formed, the proper installation of the elastic piece is completed when the installation of the tubular portion is completed. As a result, assembly when installing the bump foil can be improved.

[0009] In some examples, the bump foil may include a connecting portion connecting the tubular portion and the elastic piece portion, and the elastic piece portion may include a peak edge portion formed on the circumferentially opposite side of the connecting portion and a side edge portion extending from the peak edge portion to the connecting portion. The peak edge portion may be formed with a slit structure so as to be a free end. The peak edge portion as a free end is movable around the connecting portion connected to the tubular portion as a fulcrum.

[0010] In some examples, the bump foil has a first edge that is an end on one side in the axial direction and a second edge that is opposite the first edge in the axial direction, the slit structure has a slit that forms a peak edge portion and a side slit that is connected to the slit and forms a side edge portion, the slit and the side slit may be spaced apart from the first edge and the second edge and arranged to close within the bump foil. This makes it easier to form an elastic piece portion having a desired elastic force at a position spaced apart from the first edge or the second edge of the bump foil, for example, at a position closer to the center in the axial direction.

[0011] In some examples, the bump foil may have a first edge that is an end on one side in the axial direction and a second edge that is opposite the first edge in the axial direction. The slit structure may have a slit that forms a peak edge. The slit may be open at the first edge or the second edge, and at least a portion of the side edge may be formed at the first edge or the second edge where the slit is open. The slit that forms the peak edge, which is the free end, is open at the first edge or the second edge of the bump foil. Therefore, the peak edge can be easily formed by making a cut or the like from the first edge or the second edge.

[0012] In some examples, the slit structure may include a slit that forms a peak edge. The elastic piece portion may include a plurality of ridges that are adjacent to each other in the circumferential direction and that can expand in the circumferential direction by elastic deformation. The slit may extend between adjacent ridges, avoiding the ridges, and along the ridges. This is unlikely to impair the elastic function of the plurality of ridges that can expand in the circumferential direction by elastic deformation.

[0013] In some examples, the slit structure may include a slit that forms a peak edge portion, and a plurality of elastic pieces may be provided in a line in the circumferential direction, with the peak edge portion of one adjacent elastic piece and the peak edge portion of the other adjacent elastic piece being formed by a common slit. By sharing the slit that forms the peak edge portion of one adjacent elastic piece with the peak edge portion of the other adjacent elastic piece, the distance between adjacent elastic pieces can be reduced, making it easier to optimize the shape and dimensions of the elastic pieces, as well as the number of elastic pieces formed in the circumferential direction.

[0014] In some examples, the slit structure further includes side slits that form side edges, and the width of the slits in the circumferential direction may be greater than the width of the side slits in the axial direction. For example, if the peak edges of one elastic piece and the peak edges of the other elastic piece that are adjacent in the circumferential direction are formed by a common slit, the peak edges of the one elastic piece and the peak edges of the other elastic piece will move toward each other when the elastic pieces are deployed. Here, because the width of the slit is greater than the width of the side slits, it is easier to prevent interference between the peak edges than when the widths of the slits and the side slits are the same.

[0015] In some examples, the vehicle may further include a set foil that is arranged to surround the bump foil and supports the elastic piece portion. The set foil can prevent the elastic piece portion from protruding outward from the cylindrical portion, thereby preventing a decrease in elastic function. In addition, the provision of the set foil generates friction between the bump foil and the set foil, improving damping performance.

[0016] One example of the present disclosure is a method for manufacturing a radial foil bearing including a top foil having a surface facing a rotating shaft, a bump foil disposed outside the top foil, and a bearing housing disposed to surround the bump foil. The manufacturing method includes a step of rolling a laminated body including the top foil in an expanded state and the bump foil in an expanded state to form a cylindrical foil assembly, and a step of inserting and installing the foil assembly into the cylindrical bearing housing so as to surround the rotating shaft. The bump foil includes a cylindrical portion that cylindrically surrounds the top foil, and an elastic piece portion that is adjacent to the cylindrical portion in the axial direction of the rotating shaft, is deformable independently of the cylindrical portion, and is expandable in the circumferential direction. By inserting the foil assembly into the bearing housing, the elastic piece portion is supported by the bearing housing. This manufacturing method enables the manufacture of a radial foil bearing that can stably support a rotating shaft during high-speed rotation while improving assembly ease.

[0017] In some examples, the process of forming the foil assembly may include forming a laminate by placing the set foil in an expanded state over the bump foil on the opposite side of the top foil, and then rolling the laminate to form the foil assembly. This allows for the manufacture of a radial foil bearing that prevents a decrease in elastic function and improves damping. [Effects of the Invention]

[0018] According to some examples of the present disclosure, the rotating shaft can be stably supported when rotating at high speed. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a cross-sectional view showing an example of a rotary machine. [Figure 2] FIG. 2 is a perspective view showing an example of a radial foil bearing. [Figure 3] FIG. 3 is a cross-sectional view of the radial foil bearing taken along a plane perpendicular to the rotation axis. [Figure 4] FIG. 4 is a perspective view of the foil assembly. [Figure 5] FIG. 5 shows the bump foil in an unfolded state, with (a) being a plan view of the bump foil and (b) being a perspective view of the bump foil. [Figure 6] FIG. 6 is a schematic cross-sectional view for explaining the function of a radial foil bearing. [Figure 7] Figure 7 is an oblique view showing the process of manufacturing a radial foil bearing, where (a) is an oblique view showing the top foil, bump foil, and set foil arranged one above the other, (b) is an oblique view showing the foil assembly (laminate) before being rolled, and (c) is an oblique view showing the foil assembly being inserted into the bearing housing. [Figure 8] FIG. 8 is a schematic cross-sectional view for functionally explaining a radial foil bearing according to a modified example. [Figure 9] FIG. 9 shows a first modified example of the bump foil, and is a plan view of the bump foil in an unfolded state as seen from behind, with a portion thereof enlarged. [Figure 10] FIG. 10 shows a second modified example of the bump foil, and is a plan view of the bump foil in an unfolded state as seen from behind. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicated description will be omitted.

[0021] 1 shows a rotary machine 1. The rotary machine 1 is, for example, an electrically assisted turbocharger. The rotary machine 1 includes a turbine 2, a compressor 3, an electric motor 10, and a rotary shaft 15. The turbine 2 has a turbine impeller 4 provided at one end of the rotary shaft 15, and a turbine housing 6 that houses the turbine impeller 4. The compressor 3 has a compressor impeller 5 provided at the other end of the rotary shaft 15, and a compressor housing 7 that houses the compressor impeller 5.

[0022] A rotor 11 of the electric motor 10 is disposed, for example, at the center of a rotating shaft 15. The rotor 11 is fixed to the rotating shaft 15 and is rotatable together with the rotating shaft 15. A stator 12 of the electric motor 10 is disposed to surround the rotor 11. The stator 12 is fixed to a motor housing 13 provided between the turbine housing 6 and the compressor housing 7. The stator 12 is able to rotate the rotor 11 by generating a magnetic field around the rotating shaft 15. The cooperation of the rotor 11 and the stator 12 assists in the rotation of the rotating shaft 15.

[0023] In the rotary machine 1, exhaust gas discharged from the internal combustion engine flows into the turbine housing 6 through the scroll passage 6a and rotates the turbine impeller 4 about the rotation axis H. The exhaust gas that rotates the turbine impeller 4 is discharged through the discharge port 6b of the turbine housing 6. When the turbine impeller 4 rotates as described above, the compressor impeller 5 rotates via the rotary shaft 15. At this time, torque is applied to the rotary shaft 15 by the electric motor 10, thereby assisting the rotation of the rotary shaft 15 and the compressor impeller 5. The rotating compressor impeller 5 draws in external air through the suction port 7b of the compressor housing 7. This air is compressed as it passes through the compressor impeller 5 and the scroll passage 7a. The compressed air is discharged from the discharge port of the compressor housing 7 and supplied to the internal combustion engine.

[0024] The rotating shaft 15 is supported rotatably around the rotation axis H via a plurality of bearings. At least one of the plurality of bearings is the radial foil bearing 20 of the present disclosure. In the example shown in FIG. 1 , a pair of radial foil bearings 20 are provided at both ends of the rotating shaft 15. The radial foil bearings 20 are air bearings that support the rotating shaft 15 in the radial direction (i.e., the direction perpendicular to the rotation axis H).

[0025] In the rotary machine 1 of the present disclosure, a thrust collar 17 and a pair of thrust air bearings 18 are provided between the compressor impeller 5 and one of the pair of radial foil bearings 20 that is closer to the compressor impeller 5. The thrust collar 17 is a disk-shaped member formed to protrude like a flange around the periphery of the rotating shaft 15. The pair of thrust air bearings 18 are provided at positions sandwiching the thrust collar 17. A spacer 19 is provided between the pair of thrust air bearings 18 to surround the thrust collar 17. The thrust collar 17 and the pair of thrust air bearings 18 support the rotating shaft 15 in the thrust direction (i.e., the direction parallel to the rotation axis H).

[0026] <Radial foil bearing> Next, an example of the radial foil bearing 20 will be described in detail with reference to Figures 2 and 3. The rotary machine 1 of the present disclosure includes a pair of radial foil bearings 20, and the pair of radial foil bearings 20 have substantially the same structure. Therefore, one of the radial foil bearings 20 will be described below as a representative example.

[0027] The radial foil bearing 20 includes a foil assembly 21 and a bearing housing 45 that accommodates the foil assembly 21. The foil assembly 21 includes a top foil 22, a bump foil 23 that surrounds the top foil 22, and a set foil 24 that surrounds the bump foil 23. When the rotating shaft 15 rotates, an air film is formed between the rotating shaft 15 and the top foil 22. By forming this air film, the radial foil bearing 20 supports the rotating shaft 15 so that it can rotate freely.

[0028] <Bearing housing> The bearing housing 45 is a substantially cylindrical casing and has a shaft hole 46 through which the rotating shaft 15 is inserted. The foil assembly 21 is accommodated inside the shaft hole 46 so as to surround the rotating shaft 15. In other words, the bearing housing 45 is arranged so as to surround the bump foil 23 with the set foil 24 sandwiched between them. For example, the bearing housing 45 includes a cylindrical inner circumferential surface on the inside. This inner circumferential surface can be considered to be the outer circumferential surface of the shaft hole 46. This inner circumferential surface faces the foil assembly 21 and essentially supports the foil assembly 21.

[0029] The shaft hole 46 penetrates the bearing housing 45 in the direction in which the rotation axis H extends. In the following description, the direction in which the shaft hole 46 extends, i.e., the direction in which the rotation axis H (rotation shaft 15) extends, will be referred to as the "axial direction D1" (see FIG. 4). The radial direction of the shaft hole 46, i.e., the direction perpendicular to the rotation axis H (rotation shaft 15), will be referred to as the "radial direction D2." The circumferential direction of the rotation shaft 15, i.e., the direction along a ring centered on the rotation axis H, will be referred to as the "circumferential direction D3."

[0030] The bearing housing 45 is a member having sufficient strength to support, for example, the rotating shaft 15 when it has stopped rotating. The rotating shaft 15 is arranged to pass through the center of a shaft hole 46 in the bearing housing 45. A cylindrical foil assembly 21 is arranged between the rotating shaft 15 and an inner circumferential surface 46a of the shaft hole 46. The foil assembly 21 is provided with a locking structure 31 that protrudes in the radial direction D2, and a locking groove 46b into which the locking structure 31 is fitted is provided on the inner circumferential surface 46a of the shaft hole 46. The locking structure 31 is provided to extend in the axial direction D1, and the locking groove 46b is a vertically elongated groove that extends in the axial direction D1 so that the locking structure 31 is fitted therein. The width of the locking groove 46b in the circumferential direction D3 corresponds to the width of the locking structure 31 in the circumferential direction D3, and the locking groove 46b can abut against the locking structure 31 in the circumferential direction D3. When the foil assembly 21 attempts to move in the circumferential direction D3 of the rotation shaft 15, the locking structure 31 interferes with the locking groove 46b, and as a result, the rotation of the foil assembly 21 is prevented.

[0031] <Foil assembly> 2, 3, and 4, the foil assembly 21 includes a top foil 22, a bump foil 23, and a set foil 24. The top foil 22, the bump foil 23, and the set foil 24 are each formed from a thin metal plate and are flexible. The top foil 22, the bump foil 23, and the set foil 24 are stacked in this order to form a cylindrical shape with the top foil 22 on the inner circumferential side.

[0032] <Top foil> The cylindrical top foil 22 has an inner peripheral surface facing the rotation shaft 15 and an outer peripheral surface 22b on the opposite side of the inner peripheral surface. The inner peripheral surface is an opposing surface 22a facing the rotation shaft 15. In the unfolded state, the top foil 22 is a thin plate having a substantially rectangular shape with no slits or the like formed therein, and both longitudinal ends thereof are first hooks 31a bent in an L shape.

[0033] <Bump foil> The cylindrical bump foil 23 has an inner peripheral surface 23a facing the top foil 22 and an outer peripheral surface 23b opposite the inner peripheral surface 23a. The unfolded bump foil 23 (see FIG. 5) is a thin plate that is approximately rectangular in plan view, and both ends in the longitudinal direction D4 form second hooks 31b that are bent into an L shape. Note that the longitudinal direction D4 refers to the circumferential direction D3 when the bump foil 23 is rolled up. Therefore, in the following description, the longitudinal direction D4 and the circumferential direction D3 will be uniformly described as the circumferential direction D3, except when it is necessary to distinguish between them.

[0034] Unlike the top foil 22, the bump foil 23 has multiple peaks 32 arranged in the circumferential direction D3. The peaks 32 are intended to be protruding portions that bulge inward relative to the inner circumferential surface 23a, and valleys 33 are formed between the multiple peaks 32 on the inner circumferential surface 23a. When the outer circumferential surface 23b is used as the reference, the peaks 32 are recessed portions, and conversely, the valleys 33 are bulged portions. The peaks 32 of the present disclosure are corrugated, with concave and convex shapes curved alternately in the circumferential direction D3. However, the concave and convex shapes may not be continuous, with the concave and convex shapes interrupted midway to form peaks partially. Instead of a corrugated shape, the peaks may be convexly curved, forming a dome shape or a part of a sphere.

[0035] The peaks 32 are curved in an arc-like shape in a cross-sectional view and can expand in the circumferential direction D3 due to elastic deformation, e.g., have flexibility. This cross-sectional view is assumed to be a cross-sectional view when the bump foil 23 is cut along a plane along the circumferential direction D3. The peaks 32 are provided so as to bulge inward, facing the top foil 22. When the foil assembly 21 is formed, the peaks 32a of the arc-shaped peaks 32 can abut against the outer peripheral surface 22b of the top foil 22. The bump foil 23 according to this embodiment has multiple peaks 32 arranged in the circumferential direction D3, and the peaks 32a of each peak 32 extend in the axial direction D1. Therefore, when a load in the radial direction D2 is applied to the bump foil 23, the bump foil 23 deforms so as to expand (widen) in the circumferential direction D3 from the vicinity of the portion to which the load is applied.

[0036] The bump foil 23 includes a cylindrical portion 25, an elastic portion 26, and a connecting portion 27 that connects the cylindrical portion 25 and the elastic portion 26. The cylindrical portion 25 is a portion that cylindrically surrounds the top foil 22. The cylindrical portion 25 is, for example, a portion that is continuous without interruption from one end to the other end of the bump foil 23 in the circumferential direction D3. The cylindrical portion 25 being continuous without interruption in the circumferential direction D3 means, for example, a portion that is not divided by a slit extending in the axial direction D1 in the middle of the circumferential direction D3. Also, the cylindrical portion 25 does not necessarily have to be completely cut in the axial direction D1; therefore, it does not exclude displacement in the axial direction D1. For example, the cylindrical portion 25 may be curved in the axial direction D1 in the middle of the circumferential direction D3 between one end and the other end of the bump foil 23, or may have a structure that forms a single continuous curved shape in the circumferential direction D3.

[0037] The elastic portion 26 is integral with the tubular portion 25. The elastic portion 26 includes a first elastic piece 28A and a second elastic piece 28B extending from the joint 27 in the circumferential direction D3. The first elastic piece 28A and the second elastic piece 28B are disposed adjacent to the tubular portion 25 in the axial direction D1 and are capable of deforming independently of the tubular portion 25. Therefore, for example, when a load is applied only to the first elastic piece 28A and the second elastic piece 28B, the first elastic piece 28A and the second elastic piece 28B are significantly deformed in the circumferential direction D3, while the tubular portion 25 is barely deformed. As a result, the first elastic piece 28A and the second elastic piece 28B extend in the circumferential direction D3 more widely than the tubular portion 25.

[0038] As described below, the first elastic piece portion 28A and the second elastic piece portion 28B are formed by a slit structure 40, which allows the first elastic piece portion 28A and the second elastic piece portion 28B to deform independently of the tubular portion 25. That is, the first elastic piece portion 28A and the second elastic piece portion 28B can deform in the circumferential direction D3 without affecting each other by the width of the slit structure 40. In addition, in the present disclosure, the thickness of the metal plate that constitutes the bump foil 23 is uniform. Therefore, the first elastic piece portion 28A and the second elastic piece portion 28B are structured to be more easily deformed than the tubular portion 25.

[0039] The first elastic piece portion 28A and the second elastic piece portion 28B are supported by the cylindrical inner peripheral surface of the bearing housing 45. Note that "supported by the bearing housing 45" includes both cases where they are directly supported by contacting the bearing housing 45, and cases where they are indirectly supported by an intervening object between them and the bearing housing 45. In the present disclosure, a set foil 24 is provided as the intervening object, and the first elastic piece portion 28A and the second elastic piece portion 28B are indirectly supported by the bearing housing 45.

[0040] The first elastic piece portion 28A and the second elastic piece portion 28B are each curved to be flexible and expandable in the circumferential direction D3 to be elastically deformable. The first elastic piece portion 28A has a peak edge portion 28a formed on the opposite side of the joint portion 27 in the circumferential direction D3, and a side edge portion 28b extending from the peak edge portion 28a to the joint portion 27. The second elastic piece portion 28B has a peak edge portion 28a formed on the opposite side of the joint portion 27 in the circumferential direction D3, and a side edge portion 28b extending from the peak edge portion 28a to the joint portion 27. Here, "the opposite side of the joint portion 27 in the circumferential direction D3" refers to the position of an end portion that is opposite the joint portion 27 with respect to the circumferential direction D3.

[0041] The first elastic piece portion 28A can also be described as extending from the connecting portion 27 in one direction based on the circumferential direction D3. The second elastic piece portion 28B can also be described as extending from the connecting portion 27 in a direction opposite to the one direction based on the circumferential direction D3. Furthermore, the tip of each of the first elastic piece portion 28A and the second elastic piece portion 28B can also be described as having a peak edge portion 28a formed thereon, which is a free end that is movable with the connecting portion 27 as a fulcrum.

[0042] When the unfolded bump foil 23 (see FIG. 4) is rolled into a cylindrical shape, the first elastic piece portion 28A and the second elastic piece portion 28B are provided so as to protrude in a curved state, for example, in the tangential direction of the cylindrical portion 25. The bump foil 23 is surrounded by the set foil 24, which is, for example, a rolled-up, substantially rectangular thin plate without slits or the like. When surrounded by the set foil 24, the first elastic piece portion 28A and the second elastic piece portion 28B abut against and are pressed down by the set foil 24. As a result, the first elastic piece portion 28A and the second elastic piece portion 28B no longer protrude in a bouncy manner from the cylindrical portion 25. By abutting against the set foil 24, the first elastic piece portion 28A and the second elastic piece portion 28B are each curved in an arc shape. In addition, the first elastic piece portion 28A and the second elastic piece portion 28B elastically support the tubular portion 25 by abutting against the set foil 24 in an arc-shaped curved state, and further elastically support the top foil 22 via the tubular portion 25.

[0043] As described above, the elastic portion 26 includes the first elastic piece 28A and the second elastic piece 28B, which are curved in an arc shape. In the present disclosure, the first elastic piece 28A and the second elastic piece 28B are provided with a plurality of peaks 32. The peaks 32 are already curved in an arc shape before the bump foil 23 is rolled. However, the "arc shape" of the first elastic piece 28A and the second elastic piece 28B is not formed before the bump foil 23 is rolled, but is a shape that is understood separately from the arc shape of the peaks 32, and is formed by rolling the bump foil 23. For example, when the first elastic piece 28A and the second elastic piece 28B are understood functionally, the plurality of peaks 32 can be omitted for convenience and they can be understood as having a smooth arc shape (see FIG. 6). Functionally, the cylindrical portion 25 is similar, and can be understood as a cylindrical portion without any irregularities by omitting the multiple peaks 32. In Fig. 6, the peaks 32 are functionally simplified as elastic bodies, and are shown as, for example, simplified springs. In Fig. 8, the peaks 32 are shown as simplified springs, as in Fig. 6.

[0044] From a functional perspective, the first elastic piece 28A, from which the peaks 32 are omitted, can be assumed to be a curved plate-like portion extending from the joint 27 in the circumferential direction D3. In other words, this curved plate-like portion can be understood as a simple shape that is curved in an arc without undulations. Similarly, the second elastic piece 28B, from which the peaks 32 are omitted functionally, can be understood as a simple shape that is curved in an arc without undulations. Furthermore, the centers of curvature of the arcs of the first elastic piece 28A and the second elastic piece 28B are located inside the bump foil 23. Note that a position inside the bump foil 23 refers to a position closer to the rotation axis 15 than the bump foil 23. Furthermore, the radius of curvature of the first elastic piece 28A is larger than the radius of curvature of the tubular portion 25, and the radius of curvature of the second elastic piece 28B is larger than the radius of curvature of the tubular portion 25. The peaks 32 formed on the first elastic piece 28A and the second elastic piece 28B are arc-shaped with their centers of curvature positioned outward from the bump foil 23, and have a different technical significance from the arc-shaped first elastic piece 28A and the second elastic piece 28B. Here, the position outward from the bump foil 23 refers to an outer position that is farther from the rotation axis 15 than the bump foil 23.

[0045] Next, the slit structure 40 provided in the bump foil 23 will be described. First, the bump foil 23 has a first edge 23c which is an end on one side in the axial direction D1, and a second edge 23d which is opposite the first edge 23c in the axial direction D1. The slit structure 40 may form the first elastic piece portion 28A or the second elastic piece portion 28B independently from the first edge 23c and the second edge 23d, or may form the first elastic piece portion 28A or the second elastic piece portion 28B in cooperation with the first edge 23c or the second edge 23d.

[0046] Furthermore, the slit structure 40 functions as a boundary that separates the first elastic piece portion 28A and the second elastic piece portion 28B from the tubular portion 25, and forms the portion where the first elastic piece portion 28A and the second elastic piece portion 28B are connected to the joint portion 27. The slit structure 40 includes a first slit continuous portion 41 that forms the first elastic piece portion 28A, and a second slit continuous portion 42 that forms the second elastic piece portion 28B.

[0047] The first slit continuous portion 41 (see FIG. 5) includes a slit 43 that forms the peak edge portion 28a, and two (or more) side slits 44 that are connected to the slit 43 and form the side edge portion 28b. The slit 43 extends, for example, in the axial direction D1. The slit 43 separates the tip of the first elastic piece portion 28A from the tubular portion 25 so as to form the peak edge portion 28a. The side slits 44 extend in a direction intersecting with the slit 43. The direction intersecting with the slit 43 includes not only a direction perpendicular to the longitudinal direction of the slit 43, but also a direction inclined relative to the direction perpendicular to the slit 43. The side slits 44 are provided so as to be connected to both ends of the slit 43. The side slits 44 extend in the circumferential direction D3 so as to separate the side edge portion 28b of the first elastic piece portion 28A from the tubular portion 25.

[0048] The slits 43 and side slits 44 of the present disclosure are spaced apart from the first edge 23c and second edge 23d, which are both side edges of the bump foil 23, and do not reach the first edge 23c or the second edge 23d. In other words, the slits 43 and side slits 44 are provided so as to close within the bump foil 23 without opening at the first edge 23c or the second edge 23d.

[0049] The slits 43 are provided between adjacent peaks 32, avoiding the peaks 32. The slits 43 are provided at the positions of the valleys 33 so as to extend along the peaks 32. In other words, the peak edges 28a, which are the free ends of the first elastic piece 28A and the second elastic piece 28B, are provided along the valleys 33. By providing the peak edges 28a so as to avoid the peaks 32, it is possible to prevent the elastic function of the peaks 32 from being impaired.

[0050] The second slit continuation portion 42 has substantially the same structure as the first slit continuation portion 41, and includes a slit 43 and two (or more) side slits 44. The second slit continuation portion 42 is provided symmetrically with respect to the first slit continuation portion 41 in the circumferential direction D3 of the rotating shaft 15.

[0051] A connecting portion 27 is disposed between the first slit continuation portion 41 and the second slit continuation portion 42. The connecting portion 27 is a portion that connects and integrates the first elastic piece portion 28A and the second elastic piece portion 28B with the tubular portion 25. The distance from the connecting portion 27 to the slit 43 of the first slit continuation portion 41 is the same as the distance from the connecting portion 27 to the slit 43 of the second slit continuation portion 42. Note that the distance from the connecting portion 27 to the slit 43 of the first slit continuation portion 41 and the distance from the connecting portion 27 to the slit 43 of the second slit continuation portion 42 may be different.

[0052] In the present disclosure, a plurality of first slit continuous portions 41 and second slit continuous portions 42 are formed in the bump foil 23. As a result, a plurality of first elastic piece portions 28A and second elastic piece portions 28B are formed. Furthermore, by forming a plurality of first elastic piece portions 28A and second elastic piece portions 28B, a plurality of elastic portions 26 are formed.

[0053] As shown in FIG. 5, for example, the unfolded bump foil 23 has first slit continuous portions 41 and second slit continuous portions 42 alternately arranged along the circumferential direction D3. As a result, the unfolded bump foil 23 has first elastic piece portions 28A and second elastic piece portions 28B alternately arranged along the circumferential direction D3. A single elastic portion 26 is formed by a combination (elastic piece unit) of the first elastic piece portion 28A and the second elastic piece portion 28B adjacent to each other in the circumferential direction D3. The bump foil 23 of the present disclosure includes, for example, a first elastic piece unit 29A, a second elastic piece unit 29B, and a third elastic piece unit 29C arranged along the circumferential direction D3. When the bump foil 23 is rolled up, the first elastic piece unit 29A, the second elastic piece unit 29B, and the third elastic piece unit 29C form three (plural) elastic portions 26 aligned in the circumferential direction D3 (see FIG. 6).

[0054] The peak edge 28a of the first elastic piece unit 29A and the peak edge 28a of the second elastic piece unit 29B are formed by a common slit 43. Specifically, the slit 43 that forms the second elastic piece portion 28B of the first elastic piece unit 29A and the slit 43 that forms the first elastic piece portion 28A of the second elastic piece unit 29B are common. Furthermore, the slit 43 that forms the second elastic piece portion 28B of the second elastic piece unit 29B and the slit 43 that forms the first elastic piece portion 28A of the third elastic piece unit 29C are common. In other words, in the present disclosure, the peak edge 28a of one adjacent elastic portion 26 and the peak edge 28a of the other elastic portion 26 are formed by a common slit 43. By commonizing the peak edge 28a of one adjacent elastic portion 26 and the peak edge 28a of the other elastic portion 26 by a single slit 43, the distance between the adjacent elastic portions 26 can be reduced. As a result, it becomes easier to optimize the shape and dimensions of the elastic portions 26, as well as the number of elastic portions formed in the circumferential direction D3.

[0055] <Set Foil> As shown in Figures 3 and 4, the cylindrical set foil 24 is disposed so as to surround the bump foil 23 and supports the elastic portion 26, i.e., the first elastic piece portion 28A and the second elastic piece portion 28B. When the set foil 24 supports the elastic portion 26, it may support the first elastic piece portion 28A and the second elastic piece portion 28B in a state in which elasticity is imparted, or in a state in which elasticity is not generated. The set foil 24 has an inner peripheral surface 24a facing the bump foil 23 and an outer peripheral surface 24b opposite the inner peripheral surface 24a. In the unfolded state, the set foil 24 is a thin plate having a substantially rectangular shape, and both ends in the circumferential direction D3 are third hooks 31c bent in an L-shape.

[0056] 2 and 3, the top foil 22, bump foil 23, and set foil 24 are stacked in this order from the inner circumferential side facing the rotating shaft 15. First hooks 31a formed on both ends of the top foil 22, second hooks 31b formed on both ends of the bump foil 23, and third hooks 31c formed on both ends of the set foil 24 are arranged to overlap each other and form a locking structure 31. The locking structure 31 is housed in a locking groove 46b of a bearing housing 45.

[0057] <Modified example of radial foil bearing> FIG. 8 is a schematic cross-sectional view for functionally explaining a radial foil bearing according to a modified example. The radial foil bearing 20A according to the modified example has substantially the same structure and components as the radial foil bearing 20 described above. Therefore, the same structures and components are denoted by the same reference numerals and detailed descriptions are omitted. The radial foil bearing 20A has a foil assembly 21A and a bearing housing 45 that accommodates the foil assembly 21A. The foil assembly 21A does not have a set foil 24, but is formed by a top foil 22 and a bump foil 23. The elastic portion 26 of the bump foil 23 is in direct contact with the bearing housing 45 to elastically support the cylindrical portion 25, and further elastically supports the top foil 22 via the cylindrical portion 25. The elastic portion 26 bends while in direct sliding contact with the bearing housing 45.

[0058] <Modified bump foil> Fig. 9 illustrates a bump foil 23A according to a first modified example, and is a plan view showing an enlarged portion of the outer peripheral surface 23b of the bump foil 23A in an unfolded state. Fig. 10 illustrates a bump foil 23B according to a second modified example, and is a plan view showing the outer peripheral surface 23b of the bump foil 23B in an unfolded state. Note that the bump foil 23A according to the first modified example and the bump foil 23B according to the second modified example can be applied to the above-mentioned radial foil bearings 20 and 20A, respectively.

[0059] The bump foil 23A has a slit structure 40A that separates the first elastic piece 28C and the second elastic piece 28D from the tubular portion 25 so that they can deform independently. In the first elastic piece 28C and the second elastic piece 28D that are adjacent in the circumferential direction D3, the peak edge 28a of the first elastic piece 28C and the peak edge 28a of the second elastic piece 28D are formed by a common slit 43A. The width Wa of the slit 43A is larger than the width Wb of the side slit 44A. The width Wa of the slit 43A refers to the width of the bump foil 23 in the circumferential direction D3, and refers to the width in the circumferential direction D3 when the bump foil 23 is rolled up. The width Wb of the side slit 44A refers to the width in the axial direction D1.

[0060] Peak edge 28a of first elastic piece 28C and peak edge 28a of second elastic piece 28D move toward each other when first elastic piece 28C and second elastic piece 28D bend and expand. Here, width Wa of slit 43A forming peak edge 28a is larger than width Wb of side slit 44A, making it easier to prevent interference between peak edge portions 28a compared to when the width of slit 43 and the width of side slit 44 are the same.

[0061] As shown in FIG. 10 , a bump foil 23B according to the second modification includes a first edge 23c at one end in the axial direction D1 and a second edge 23d on the opposite side of the first edge 23c in the axial direction D1. The tubular portion 25B of the bump foil 23B is located at a position spaced apart from the first edge 23c and the second edge 23d. For example, the tubular portion 25B includes a central region between the first edge 23c and the second edge 23d and is provided so as to extend in the circumferential direction D3 (the circumferential direction D3). The bump foil 23B includes a first elastic piece 28E and a second elastic piece 28F adjacent to the tubular portion 25B in the axial direction D1. That is, the first elastic piece 28E and the second elastic piece 28F are provided between the first edge 23c or the second edge 23d and the tubular portion 25B.

[0062] The bump foil 23B has a slit structure 40B that separates the first elastic piece 28E and the second elastic piece 28F from the tubular portion 25B so that they can deform independently. The slit structure 40B has a slit 43B that forms the peak edge 28a and a side slit 44B that is connected to the slit 43B and forms the side edge 28b. The slit 43B is open at the first edge 23c or the second edge 23d.

[0063] At least a portion of the side edge portions 28b of the first elastic piece portion 28E and the second elastic piece portion 28F is formed on the first edge 23c or the second edge 23d. Specifically, of the pair (plurality) of side edge portions 28b forming the first elastic piece portion 28E, some of the side edge portions 28b are formed on the first edge 23c or the second edge 23d, and the other side edge portions 28b are formed by the side slits 44B. Also, of the pair (plurality) of side edge portions 28b forming the second elastic piece portion 28F, some of the side edge portions 28b are formed on the first edge 23c or the second edge 23d, and the other side edge portions 28b are formed by the side slits 44B. That is, in the case of the bump foil 23B according to the second modified example, the slit structure 40B cooperates with the first edge 23c or the second edge 23d to form the first elastic piece portion 28E or the second elastic piece portion 28F.

[0064] <Manufacturing method for radial foil bearings> Next, a manufacturing method of the radial foil bearing 20 will be described with reference to Fig. 7. The manufacturing method of the present disclosure includes a foil forming step, a foil assembly forming step, and a foil assembly installing step.

[0065] The foil forming process includes a top foil forming process, a bump foil forming process, and a set foil forming process. In the top foil forming process, a first metal plate-shaped member is processed into a predetermined shape (e.g., a rectangle) to form the top foil 22 in an expanded state. Both ends of the top foil 22 in the circumferential direction D3 are bent to form the first hooks 31a. Note that the set foil forming process can be omitted in the manufacturing method for the radial foil bearing 20A according to the above-described modified example. Furthermore, the foil forming process can also be omitted when using the top foil 22, bump foils 23, 23A, 23B, and set foil 24 that have been processed into the desired shapes.

[0066] In the bump foil forming process, a second metal plate-shaped member is processed into a predetermined shape (e.g., rectangular) to form the unfolded bump foil 23. Both ends of the bump foil 23 in the circumferential direction D3 are bent to form second hooks 31b. Furthermore, a slit structure 40 is formed in the bump foil 23 to form the tubular portion 25 and the elastic portion 26. The elastic portion 26 includes first elastic piece portions 28A, 28C, and 28E and second elastic piece portions 28B, 28D, and 28F. Furthermore, the bump foil 23 is processed to form multiple arc-shaped peaks 32. In the present disclosure, by forming the slit structure 40, the tubular portion 25 and the elastic portion 26 can be integrally formed when the bump foil 23 is rolled. In other words, it is not necessary to form the elastic portion 26 by combining multiple members. The slit structure 40 may be formed before or after the peaks 32 are formed.

[0067] In the set foil forming process, a third metal plate-shaped member is processed to have a predetermined shape (e.g., a rectangle) to form the unfolded set foil 24. Both ends of the set foil 24 in the circumferential direction D3 are bent to form the third hooks 31c.

[0068] 7(b) and 7(c), in the foil assembly forming process, a laminate of the top foil 22 in an unfolded state, the bump foil 23 in an unfolded state, and the top foil 22 in an unfolded state is rolled up to form a tubular foil assembly 21. When forming the foil assembly 21, the first hook 31a, the second hook 31b, and the third hook 31c are stacked to form the locking structure 31. When the bump foil 23 is rolled up to form the foil assembly 21, the elastic portion 26 is formed by the slit structure 40.

[0069] In the foil assembly installation process, the foil assembly 21 is inserted into the bearing housing 45 so as to surround the rotating shaft 15, thereby forming the radial foil bearing 20. Specifically, the foil assembly 21 is inserted into the axial hole 46 of the bearing housing 45. A ring-shaped pressing member or the like is installed at the end of the bearing housing 45 in the axial direction D1 (see FIG. 1 ) to restrict displacement of the foil assembly 21 in the axial direction D1.

[0070] Next, the effects achieved by the radial foil bearings 20, 20A of the present disclosure will be described.

[0071] The bump foils 23, 23A, 23B of the radial foil bearings 20, 20A include a cylindrical portion 25 and elastic piece portions 28A to 28F. The elastic piece portions 28A to 28F are deformable independently of the cylindrical portion 25. The elastic piece portions 28A to 28F are directly or indirectly supported by the bearing housing 45, and support the top foil 22 via the cylindrical portion 25 adjacent to them in the axial direction D1. Furthermore, the elastic piece portions 28A to 28F are expandable in the circumferential direction D3 while being directly or indirectly supported by the bearing housing 45.

[0072] Elastic piece portions 28A-28F of radial foil bearing 20 flex while indirectly making sliding contact with bearing housing 45, while elastic piece portions 28A-28F of radial foil bearing 20A flex while making direct sliding contact with bearing housing 45. When elastic piece portions 28A-28F are elastically deformed and expanded while being indirectly or directly supported by bearing housing 45, they elastically support cylindrical portion 25 and further elastically support top foil 22 via cylindrical portion 25. As a result, radial foil bearings 20, 20A are more resistant to vibrations of rotating shaft 15 rotating at high speed, enabling stable support of rotating shaft 15.

[0073] Furthermore, in the present disclosure, the elastic piece portions 28A to 28F and the tubular portion 25 are integrally formed, and therefore, appropriate installation of the elastic piece portions 28A to 28F is completed once installation of the tubular portion 25 is completed. As a result, assembly can be facilitated when installing the bump foils 23, 23A, and 23B.

[0074] The bump foils 23, 23A, 23B also include a connecting portion 27 that connects the elastic piece portions 28A to 28F to the tubular portion 25. The elastic piece portions 28A to 28F each include a peak edge portion 28a formed on the opposite side of the connecting portion 27 in the circumferential direction D3, and a side edge portion 28b extending from the peak edge portion 28a to the connecting portion 27. The peak edge portion 28a is formed by slit structures 40, 40A, 40B so as to be a free end, and is therefore movable with the connecting portion 27 as a fulcrum.

[0075] The elastic piece portions 28A-28F elastically support the rotating shaft 15 via the top foil 22 by expanding themselves. The elastic piece portions 28A-28F also include a plurality of adjacent peaks 32 aligned in the circumferential direction D3. The plurality of peaks 32 are elastically deformable and expandable in the circumferential direction D3. Since the radial foil bearings 20, 20A include a plurality of peaks 32, the top foil 22 can be elastically supported by the expansion of each of the plurality of peaks 32 in addition to the expansion of the elastic piece portions 28A-28F. The slits 43, 43A, 43B forming the peak edge portions 28a of the elastic piece portions 28A-28F are provided between adjacent peaks 32, avoiding the peaks 32. As a result, the elastic function of the plurality of peaks 32, which are elastically deformable and expandable in the circumferential direction D3, is unlikely to be impaired.

[0076] Furthermore, the radial foil bearings 20, 20A of the present disclosure include set foils 24 that support the first elastic piece portions 28A, 28C, 28E and the second elastic piece portions 28B, 28D, 28F. The set foils 24 can prevent the first elastic piece portions 28A, 28C, 28E and the second elastic piece portions 28B, 28D, 28F from bouncing outward from the cylindrical portion 25, thereby preventing a decrease in elastic function. Furthermore, the provision of the set foils 24 generates friction between the bump foils 23 and the set foils 24, improving damping performance.

[0077] Furthermore, in the first elastic piece portions 28A, 28C, 28E and the second elastic piece portions 28B, 28D, 28F that are adjacent to each other in the circumferential direction D3, the peak edge portions 28a of the first elastic piece portions 28A, 28C, 28E and the peak edge portions 28a of the second elastic piece portions 28B, 28D, 28F are formed by common slits 43, 43A, 43B. By sharing the slits 43A, 43B, the distance between the adjacent first elastic piece portions 28A, 28C, 28E and the second elastic piece portions 28B, 28D, 28F can be reduced, making it easier to optimize the shapes and dimensions of the first elastic piece portions 28A, 28C, 28E and the second elastic piece portions 28B, 28D, 28F, as well as the number of elastic piece portions formed in the circumferential direction D3.

[0078] Furthermore, in the bump foil 23A according to Modification 1, the width Wa of the slit 43A in the circumferential direction D3 is greater than the width Wb of the side slit 44A in the axial direction D1. For example, if the peak edge 28a of the first elastic piece 28C and the peak edge 28a of the second elastic piece 28D are formed by a common slit 43A, when the first elastic piece 28C or the second elastic piece 28D is deployed, the peak edges 28a of the first elastic piece 28C or the second elastic piece 28D move toward each other. Here, because the width Wa of the slit 43A is greater than the width Wb of the side slit 44A, it is easier to prevent interference between the peak edges 28a compared to when the widths of the slit and the side slit are the same.

[0079] Furthermore, in the bump foil 23, 23A, the slits 43, 43B and side slits 44, 44B of the slit structure 40, 40A are provided spaced apart from the first end edge 23c and the second end edge 23d of the bump foil 23, 23A. The slits 43, 43B and the side slits 44, 44B are provided so as to be closed within the bump foil 23, 23A. As a result, it is easy to form the first elastic piece portions 28A, 28C and the second elastic piece portions 28B, 28D having the desired elastic force at positions spaced apart from the first end edge 23c or the second end edge 23d, for example, at positions closer to the center in the axial direction D1.

[0080] The slit structure 40B of the bump foil 23B according to the second modification includes a slit 43B that forms the peak edge 28a, and the slit 43B is open at the first edge 23c or the second edge 23d. As a result, the peak edge 28a can be easily formed by making a cut or the like from the first edge 23c or the second edge 23d.

[0081] Furthermore, the multiple peaks 32 are provided not only on the elastic piece portions 28A-28F but also on the connecting portion 27. The connecting portion 27 is less likely to bend than the elastic piece portions 28A-28F. In contrast, the elastic piece portions 28A-28F are more likely to bend as they move away from the connecting portion 27 in the circumferential direction D3 and closer to the peak edge portion 28a. The elastic support force of the peaks 32 on the top foil 22 decreases in the more easily bent portions of the elastic piece portions 28A-28F. Therefore, this elastic support force is greatest at the connecting portion 27 and decreases closer to the peak edge portion 28a of each of the elastic piece portions 28A-28F. In other words, the elastic support force supporting the top foil 22 by the multiple peaks 32 is not uniform in the circumferential direction D3 but varies. By arranging the multiple peaks 32 so that the elastic support force varies, the stability of the top foil 22, which vibrates due to interference with the rotating shaft 15, can be improved, thereby improving stability when supporting the rotating shaft 15 rotating at high speed.

[0082] Furthermore, in the above-described method for manufacturing the radial foil bearing 20, in the foil assembly forming step, the top foil 22, the bump foils 23, 23A, 23B, and the set foil 24 are rolled to form the foil assembly 21, and in the foil assembly installing step, the foil assembly 21 is inserted and installed in the bearing housing 45 so as to surround the rotating shaft 15. This manufacturing method improves the ease of assembly when manufacturing the radial foil bearing 20. Furthermore, the radial foil bearing 20 manufactured by this manufacturing method can stably support the rotating shaft 15 when it rotates at high speed.

[0083] In the above manufacturing method, the foil assembly 21 is formed by rolling the expanded top foil 22, the expanded bump foil 23, and the expanded set foil 24 in a stacked state. However, it is also possible to manufacture a radial foil bearing 20A without the set foil 24. When manufacturing the radial foil bearing 20A, the expanded top foil 22 and the expanded bump foil 23 are stacked and rolled to form the foil assembly 21A. In the foil assembly installation process, the foil assembly 21A is inserted into the bearing housing 45 so as to surround the rotating shaft 15.

[0084] The radial foil bearing according to the present disclosure is not limited to the above-described examples, and various other modifications are possible. For example, the above-described examples may be combined with each other depending on the required purpose and effect. Furthermore, the elastic piece portion may be configured to omit multiple peaks. For example, the first elastic piece portion may be a single arc-shaped plate portion as a whole, without including multiple arc-shaped peaks. Furthermore, the second elastic piece portion may be a single arc-shaped plate portion as a whole, without including multiple arc-shaped peaks. [Explanation of symbols]

[0085] 15 Rotation axis 20,20A Radial Foil Bearing 21,21A Foil Assembly 45 bearing housing 22 Top Foil 23, 23A, 23B Bump foil 23c First edge 23d Second edge 24 Set Foils D1 Axial direction D3 Circumferential direction 22a Opposite surface 32 Yamabe 25 Cylindrical part 27 Joint 28A, 28C, 28E First elastic piece 28B, 28D, 28F Second elastic piece 28a Peak edge 28b Side edge 40, 40A, 40B slit structure 43, 43A, 43B slits 44, 44A, 44B Side slit Wa slit width Wb Side slit width

Claims

1. A radial foil bearing, a top foil having an opposing surface facing the rotation shaft; a bump foil disposed on the outer side opposite to the opposing surface of the top foil; a bearing housing arranged to surround the bump foil; The bump foil is a cylindrical portion that cylindrically surrounds the top foil; an elastic piece portion adjacent to the cylindrical portion in the axial direction of the rotation shaft, deformable independently from the cylindrical portion, and expandable in a circumferential direction; a connecting portion that connects the tubular portion and the elastic piece portion, the elastic piece portion includes a mountain portion that can be expanded in the circumferential direction by elastic deformation, the connecting portion includes a ridge portion that can be expanded in the circumferential direction by elastic deformation, The elastic piece portion is supported by the bearing housing.

2. The elastic piece and the cylindrical portion are integrally formed, 2. The radial foil bearing according to claim 1, wherein the bump foil has a slit structure that separates the elastic piece portion from the cylindrical portion so that the elastic piece portion can deform independently from the cylindrical portion.

3. the elastic piece portion includes a peak edge portion formed on the opposite side of the joint portion in the circumferential direction, and a side edge portion extending from the peak edge portion to the joint portion, The radial foil bearing according to claim 2 , wherein the peak edge is formed by the slit structure so as to be a free end.

4. the bump foil has a first edge which is an end on one side in the axial direction, and a second edge which is opposite to the first edge in the axial direction, the slit structure comprises a slit forming the peak edge portion and a side slit connected to the slit and forming the side edge portion; The radial foil bearing according to claim 3 , wherein the slit and the side slit are spaced apart from the first edge and the second edge and are closed within the bump foil.

5. the bump foil has a first edge which is an end on one side in the axial direction, and a second edge which is opposite to the first edge in the axial direction, the slit structure comprises a slit that forms the peak edge; the slit is open at the first edge or the second edge; The radial foil bearing according to claim 3 , wherein at least a portion of the side edge portion is formed on the first end edge or the second end edge where the slit is opened.

6. the slit structure comprises a slit that forms the peak edge; The elastic piece portion includes a plurality of the ridge portions arranged adjacent to each other in the circumferential direction, The radial foil bearing according to claim 3 , wherein the slits extend between adjacently arranged ridges, avoiding the ridges and along the ridges.

7. the slit structure comprises a slit that forms the peak edge; The elastic piece portion is provided in a plurality of positions aligned in the circumferential direction, 4. The radial foil bearing according to claim 3, wherein the peak edge portion of one of the adjacent elastic piece portions and the peak edge portion of the other of the adjacent elastic piece portions are formed by the common slit.

8. The slit structure further includes a side slit forming the side edge, The radial foil bearing according to claim 7 , wherein a width of the slit in the circumferential direction is greater than a width of the side slit in the axial direction.

9. The radial foil bearing according to claim 1 , further comprising a set foil arranged to surround the bump foil and supporting the elastic piece portion while imparting elasticity to the elastic piece portion.

10. A method for manufacturing a radial foil bearing including: a top foil having an opposing surface that faces a rotation shaft; a bump foil disposed outward from the top foil; and a bearing housing disposed so as to surround the bump foil, a step of rolling a stack of the unfolded top foil and the unfolded bump foil to form a tubular foil assembly; and inserting the foil assembly into a cylindrical bearing housing so as to surround the rotating shaft, the bump foil includes a cylindrical portion that cylindrically surrounds the top foil, an elastic piece portion that is adjacent to the cylindrical portion in the axial direction of the rotation shaft, that is deformable independently of the cylindrical portion, and that is expandable in the circumferential direction, and a connecting portion that connects the cylindrical portion and the elastic piece portion, the elastic piece portion includes a mountain portion that can be expanded in the circumferential direction by elastic deformation, the connecting portion includes a ridge portion that can be expanded in the circumferential direction by elastic deformation, A method for manufacturing a radial foil bearing, wherein the foil assembly is inserted into the bearing housing, so that the elastic piece portion is supported by the bearing housing.

11. 11. The method for manufacturing a radial foil bearing according to claim 10, wherein in the step of forming the foil assembly, the laminate is formed by arranging an expanded set foil so as to overlap the bump foil on the side opposite to the top foil, and the laminate is rolled up to form the foil assembly.

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