Foil for radial foil bearing, and radial foil bearing

Abstract

A top foil (20) of a radial foil bearing (5) is provided with a foil body (21) that has a length in a first direction which enables rolling up into a tubular shape. The foil body (21) includes a pair of curved parts (21a, 21a) that are spaced apart in the first direction and an intermediate part (21b) that is positioned between the pair of curved parts (21a, 21a) and that extends from one of the pair of curved parts (21a, 21a) to the other. In a load-less state in which the foil body (21) is receiving no external force for rolling up thereof, the pair of curved parts (21a, 21a) are curved at a smaller curvature radius than the intermediate part (21b).

Description

Radial foil bearing foils and radial foil bearings

[0001] The present disclosure relates to foils of radial foil bearings and radial foil bearings.

[0002] A radial foil bearing is a type of journal bearing that rotatably supports a rotating shaft and has a foil surrounding the outer periphery of the rotating shaft as a bearing surface. The foil is elastically displaceable in the radial direction, and when the rotating shaft rotates at low speeds, it contacts the rotating shaft and elastically supports its load. On the other hand, when the rotating shaft rotates at high speeds, the wedge effect of a fluid film formed between the rotating shaft and the foil supports the rotating shaft without contacting the foil. Therefore, radial foil bearings are generally known to have higher durability and lower noise due to vibration and friction than rolling bearings.

[0003] As a related art, Patent Document 1 discloses a radial foil bearing that includes a top foil that serves as a bearing surface and a back foil that elastically supports the top foil.

[0004] JP 2023-051044 A

[0005] Non-circular journal bearings have a bearing surface whose curvature changes along the circumferential direction. In non-circular journal bearings, the spring characteristics of the fluid film become anisotropic. As a result, compared to circular journal bearings, they are able to suppress phenomena such as oil whip and stabilize the position of the rotating shaft.

[0006] The foils that make up a radial foil bearing can have a non-circular cross section. In other words, the radial foil bearing can be configured as the non-circular journal bearing described above. In a radial foil bearing, the rotating shaft is in partial contact with the foil while the rotating shaft is stationary or rotating at a relatively slow speed. Meanwhile, in order for the rotating shaft to completely separate from the foil, the rotation of the rotating shaft must be accelerated, increasing the internal pressure of the fluid film. In other words, it takes a certain amount of time for the desired internal pressure to be achieved. However, if the contact time between the rotating shaft and the foil is prolonged, the foil is more likely to wear, shortening the life (usable period) of the bearing.

[0007] The present disclosure has been made in consideration of the above-mentioned circumstances, and aims to provide a foil for a radial foil bearing and a radial foil bearing that can extend the life of the bearing.

[0008] The foil of a radial foil bearing according to a first aspect of the present disclosure comprises a foil body having a length in a first direction that allows it to be rolled into a cylindrical shape, the foil body including a pair of curved portions spaced apart from each other in the first direction, and an intermediate portion located between the pair of curved portions and extending from one of the pair of curved portions to the other, and in an unloaded state in which the foil body is not subjected to an external force to be rolled, the pair of curved portions are curved with a radius of curvature that is smaller than the radius of curvature of the intermediate portion.

[0009] The intermediate portion may be entirely flat in the unloaded state, and the distance between the pair of curved portions may be set to at least half of the entire length of the foil body along the first direction. At least a portion of the intermediate portion may be flat in the unloaded state. The foil may include a pair of flanges provided at both ends of the foil body in the first direction and folded back in a direction opposite to the direction in which the foil body is rolled.

[0010] The foil body may be formed as a top foil or an intermediate foil. The foil body may be formed as a bump foil.

[0011] A radial foil bearing according to a second aspect of the present disclosure includes the foil according to the first aspect.

[0012] According to the present disclosure, it is possible to provide a foil of a radial foil bearing and a radial foil bearing that can extend the life of the bearing while maintaining its performance as a bearing.

[0013] FIG. 1 is a side view of an example of a turbomachine to which the radial foil bearing according to the present embodiment is applied. FIG. 2 is a front view of the radial foil bearing according to the present embodiment. FIG. 3 is a side view of a top foil to which the foil according to the present embodiment is applied. FIG. 4A is a plan view of an example of a top foil. FIG. 4B is a plan view of another example of a top foil. FIG. 5A is a side view of a bump foil. FIG. 5B is a partially enlarged side view of a foil body of the bump foil. FIG. 6A is a diagram for explaining an example of a process for rolling the top foil and the bump foil into a cylindrical shape. FIG. 6B is a diagram for explaining an example of a process for rolling the top foil and the bump foil into a cylindrical shape. FIG. 6C is a diagram for explaining an example of a process for rolling the top foil and the bump foil into a cylindrical shape. FIG. 6D is a diagram for explaining an example of a process for rolling the top foil and the bump foil into a cylindrical shape. FIG. 6E is a diagram for explaining an example of a process for rolling the top foil and the bump foil into a cylindrical shape. FIG. 6F is a diagram for explaining an example of a process for rolling the top foil and bump foil into a cylindrical shape. FIG. 7 is a schematic diagram of a radial foil bearing assumed in the numerical analysis. FIG. 8A is a side view of the top foil assumed in the first numerical analysis. FIG. 8B is a graph showing the results of the numerical analysis when each of the top foils shown in FIG. 8A is used. FIG. 9A is a side view of the top foil according to the present embodiment and the top foil according to the comparative example. FIG. 9B is a graph showing the analysis results when the top foil shown in FIG. 9A is used. FIG. 10A is a side view of the top foil according to the present embodiment and the top foil according to the comparative example. FIG. 10B is a graph showing the analysis results when the top foil shown in FIG. 10A is used. FIG. 11 is a side view of a bump foil to which the foil according to the present embodiment is applied.

[0014] Several embodiments of the present disclosure will be described below. Note that common parts in each drawing are denoted by the same reference numerals, and duplicated explanations will be omitted. For ease of explanation, the terms "axial direction," "circumferential direction," and "radial direction" are defined as follows:

[0015] The "axial direction" is the extension direction of a reference axis 7 (see FIG. 1). The reference axis 7 corresponds to the central axis of the shaft 2, the thrust bearing 4, and the radial foil bearing 5. The reference axis 7 also corresponds to the central axis of an insertion hole 11 provided in the radial foil bearing 5.

[0016] The "circumferential direction" is the direction of extension of a circle on an imaginary plane that has a center on a point on the reference axis 7 and is perpendicular to the reference axis 7. The "radial direction" is the direction that starts from an arbitrary point on the reference axis 7 and extends on a plane that is perpendicular to the reference axis 7.

[0017] For the foil according to this embodiment, a "first direction A" and a "second direction B" are also defined. The "first direction A" and the "second direction B" correspond to the circumferential direction and the axial direction, respectively, when the foil according to this embodiment (see the top foil 20 shown in Figures 3 to 4B) is rolled around the reference axis 7. The "plane direction" is a direction perpendicular to an imaginary plane including the first direction A and the second direction B.

[0018] First, a turbomachine 1 to which a radial foil bearing 5 according to this embodiment is applied will be described. Fig. 1 is a side view of an example of the turbomachine 1. The turbomachine 1 includes a shaft 2, an impeller 3, a thrust bearing 4, a pair of radial foil bearings 5, 5, and a housing 6 that accommodates these. Note that Fig. 1 shows one of the pair of radial foil bearings 5, 5. Hereinafter, for convenience of explanation, the radial foil bearing 5 will be simply referred to as a bearing 5.

[0019] The bearing 5 rotatably supports the shaft 2. The impeller 3 is attached to the end of the shaft 2. The impeller 3 is housed in the housing 6 with a tip clearance formed between it and the housing 6. The thrust bearing 4 includes a thrust collar 4a and a pair of bearing pads 4b. The thrust collar 4a is a circular plate having a predetermined thickness in the axial direction Z and is fixed to the shaft 2. The thrust collar 4a is sandwiched between the pair of bearing pads 4b. This limits the range of movement of the thrust collar 4a along the axial direction Z.

[0020] Figure 2 is a front view of the bearing 5. As shown in Figure 2, the bearing 5 includes a bearing housing 10, a top foil 20, and a bump foil 30. The bearing housing 10 of this embodiment is made of metal. Below, an example in which the foil of this embodiment is applied to the top foil 20 will be described.

[0021] The bearing housing 10 is an accommodating member that accommodates at least the top foil 20 and the bump foil 30. An insertion hole 11 is provided in the bearing housing 10. The insertion hole 11 extends axially and passes through the bearing housing 10. The shaft 2 is inserted through the insertion hole 11 with the top foil 20 and the bump foil 30 accommodated in the insertion hole 11. In other words, the bump foil 30, the top foil 20, and the outer peripheral surface of the shaft 2 are arranged in this order from the insertion hole 11 toward the inside in the radial direction. As shown in FIG. 2 , the bearing housing 10 has a cylindrical outer shape. However, as long as the insertion hole 11 is formed, the outer shape of the bearing housing 10 is arbitrary.

[0022] A groove 12 is formed in the inner circumferential surface 11a of the insertion hole 11. The groove 12 opens to the inner circumferential surface 11a and extends from one end face of the bearing housing 10 to the other end face in the axial direction. The groove 12 accommodates the flanges 22, 22 of the top foil 20 and the flanges 32, 32 of the bump foil 30. The width of the groove 12 along the circumferential direction is set to a value that allows the flanges 22, 22 to be aligned circumferentially with a gap therebetween and the flanges 32, 32 to be aligned circumferentially with a gap therebetween.

[0023] The top foil 20 is a single metal foil (thin metal plate) that surrounds almost the entire circumference of the shaft 2 and has a thickness that allows elastic deformation (flexibility). The top foil 20 includes a foil body 21 that faces the shaft 2. The foil body 21 extends circumferentially along the cylindrical shape of the inner peripheral surface 11a of the insertion hole 11. Unlike the foil body 31 of the bump foil 30 described below, the foil body 21 does not have alternating concave and convex portions. That is, the surface of the foil body 21 in this embodiment does not have concave and convex portions formed by machining and functions as a bearing surface that faces the shaft 2. The top foil 20 is placed in the insertion hole 11 in a cylindrical state so as to surround the outer periphery of the shaft 2 and is located radially inward of the bump foil 30.

[0024] The bump foil 30 is a single metal foil (thin metal plate) that surrounds almost the entire circumference of the shaft 2 and has a thickness that allows elastic deformation (flexibility). However, this value is also set appropriately depending on the desired elastic force, material, etc. The bump foil 30 includes a foil main body 31 that surrounds the outer circumference of the shaft 2. The foil main body 31 extends in the circumferential direction along the cylindrical shape of the inner circumferential surface 11a of the insertion hole 11. The foil main body 31 is formed in a wavy shape that extends in the first direction A (circumferential direction) while meandering in the surface direction (radial direction) of the foil main body 31. The bump foil 30 is placed in the insertion hole 11 in a rolled cylindrical state so as to surround the outer circumference of the shaft 2 and elastically supports the top foil 20.

[0025] An intermediate foil (not shown) may be provided between the bump foil 30 and the top foil 20. An auxiliary foil (see FIG. 6A) 40 may be provided between the inner peripheral surface 11a of the insertion hole 11 and the bump foil 30. Like the top foil 20, both the intermediate foil and the auxiliary foil may be metal foils (thin plates) without irregularities, and may have a thickness that allows elastic deformation (flexibility).

[0026] The top foil 20 will be described in detail. FIG. 3 is a side view of the top foil 20 to which the foil according to this embodiment is applied. FIG. 4A is a plan view of an example of the top foil 20. FIG. 4B is a plan view of another example of the top foil 20. As described above, the top foil 20 includes a foil main body 21 formed flat in the first direction A and the second direction B. The foil main body 21 has a rectangular outer shape and a length in the first direction A (circumferential direction) that allows it to be rolled into a cylindrical shape. Furthermore, the length along the second direction B (axial direction) is approximately equal to the axial length of the bearing housing 10. When the top foil 20 is attached to the insertion hole 11, the foil main body 21 functions as a bearing surface for the shaft 2.

[0027] 3, the foil body 21 includes a pair of curved portions 21 a, 21 a and an intermediate portion 21 b. The two curved portions 21 a, 21 a are spaced apart from each other in the first direction A (circumferential direction) with the intermediate portion 21 b sandwiched therebetween. The curved portion 21 a is pre-curved in the direction in which the top foil 20 is rolled by plastic deformation using machining such as bending.

[0028] Here, the state in which the foil body 21 is not subjected to an external force for rolling is referred to as the "unloaded state." FIG. 3 shows the foil body 21 in the unloaded state. In this state, the radius of curvature R of the curved portion 21 a is smaller than the radius of curvature of the intermediate portion 21 b. The radius of curvature R of the curved portion 21 a may be constant or may gradually change. The state in which the foil body 21 is not subjected to an external force for rolling may be, for example, the state in which the foil body 21 is not subjected to an external force for rolling before being placed in the insertion hole 11 and before being rolled in the process of manufacturing the radial foil bearing 5. The state in which the foil body 21 placed in the insertion hole 11 is removed and the resistance from the inner circumferential surface 11 a of the insertion hole 11 is removed is also the state in which the foil body 21 is not subjected to an external force for rolling.

[0029] The intermediate portion 21b is located between the pair of curved portions 21a, 21a. The intermediate portion 21b extends in the first direction A from one of the pair of curved portions 21a, 21a to the other. The radius of curvature of the intermediate portion 21b in the no-load state is set to, for example, infinity. That is, in this case, the intermediate portion 21b is formed in a planar shape extending in the first direction A and the second direction B as shown in FIG. 3, and only the portions of the foil body 21 on both sides in the first direction A are pre-bent.

[0030] Note that at least a portion of the intermediate portion 21b may be formed in a flat shape. When the top foil 20 is attached to the insertion hole 11, it is conceivable that an assembly robot such as a picking robot is used to roll the top foil 20 into a cylindrical shape. In this case, if the intermediate portion 21b has a flat portion, that portion functions as a mounting surface and can stabilize the posture and position of the top foil 20. Therefore, it becomes possible to smoothly execute a series of automatic operations by the assembly robot, such as bending the top foil 20 and attaching it to the insertion hole 11, and the efficiency of assembling the bearing 5 can be improved.

[0031] The top foil 20 includes a pair of flanges 22, 22. The flanges 22 are provided at both ends of the foil body 21 in the first direction A (circumferential direction) and extend in the second direction B (axial direction). The flanges 22 protrude from the foil body 21 toward the bearing housing 10 (radially outward). In this embodiment, the flanges 22 are formed into a hollow cylindrical shape by folding back (rolling) a portion of the foil constituting the top foil 20 in the direction opposite to the direction in which the foil body 21 is rolled. A pin 50 (see FIG. 6B ) of an assembly robot is inserted into the space inside the flanges 22. The top foil 20 can be rolled by operating the pin 50 inserted into the flanges 22 (see FIGS. 6A to 6F ). In other words, the top foil 20 can be automatically attached to the insertion hole 11.

[0032] When the top foil 20 is rolled into a cylindrical shape, the flanges 22 are located radially outward of the foil body 21. Therefore, when the rolled top foil 20 is attached to the insertion hole 11, the pair of flanges 22 are housed in the grooves 12. This restricts excessive circumferential displacement (rotation) of the top foil 20 (foil body 21). In this embodiment, the flanges 22 and the bottom (the radially outer boundary) of the grooves 12 are located apart from each other. Therefore, in this embodiment, the flanges 22 do not support the radial load of the shaft 2.

[0033] 4A and 4B, the flange 22 has a portion cut out in the second direction B (axial direction). For example, as shown in Fig. 4A, the flange 22 has a central portion cut out in the second direction B (axial direction). Alternatively, as shown in Fig. 4B, the flange 22 is cut out except for the central portion in the second direction B (axial direction). When the top foil 20 is attached to the insertion hole 11, the flange 32 of the bump foil 30 is positioned in this cut-out portion.

[0034] The bump foil 30 will now be described in detail. Fig. 5A is a side view of the bump foil 30. Fig. 5B is a partially enlarged side view of the foil body 31 of the bump foil 30.

[0035] 5A , the bump foil 30 includes a foil body 31. Like the foil body 21 of the top foil 20, the foil body 31 also has a rectangular outer shape and a length in the first direction A (circumferential direction) that allows it to be rolled into a cylindrical shape. Furthermore, the length in the second direction B (axial direction) is approximately equal to the axial length of the bearing housing 10. When the bump foil 30 is attached to the insertion hole 11, the foil body 31 is positioned between the foil body 21 of the top foil 20 and the inner circumferential surface 11 a of the insertion hole 11.

[0036] The foil body 31 is formed in a wavy shape that extends in a first direction A (circumferential direction) while meandering in a surface direction (radial direction) of the foil body 31. As an example of configuring this wavy shape, the foil body 31 includes bump portions 33 and connecting portions 34 that are alternately arranged in the first direction A (circumferential direction).

[0037] The bump portion 33 has an arch-shaped cross section that protrudes to one side in the planar direction (radially inward), and elastically deforms in response to the load of the shaft 2 applied via the top foil 20. Meanwhile, the connecting portion 34 extends in the circumferential direction and connects the base portions 33a, 33a of two adjacent bump portions 33, 33. The base portions 33a are the portions of the bump portions 33 that are located most radially outward when the bump foil 30 is rolled. When the bump foil 30 is subjected to a radially outward load, the connecting portion 34 comes into contact with the inner circumferential surface 11a of the insertion hole 11 or the auxiliary foil 40 (see FIG. 6A ) and supports the bump portion 33 that receives the load.

[0038] Like the top foil 20, the bump foil 30 also includes a pair of flanges 32, 32. The position and shape of the flanges 32 are the same as those of the flanges 22 of the top foil 20. That is, the flanges 32 are provided at both ends of the foil body 31 in the first direction A (circumferential direction) and extend in the axial direction. The flanges 32 are formed by folding back (rolling) a portion of the foil that constitutes the bump foil 30 in the direction opposite to the direction in which the foil body 31 is rolled.

[0039] A pin 50 (see FIG. 6B) of an assembly robot is inserted into the space inside the flange 32. By operating the pin 50 inserted into the flange 32, the bump foil 30 can be rolled up (see FIGS. 6A to 6F). In other words, the bump foil 30 can be automatically attached to the insertion hole 11.

[0040] 6A to 6F are diagrams illustrating an example of a process for rolling the top foil 20 and the bump foil 30 into a cylindrical shape. In this example, the auxiliary foil 40 is also rolled up along with the top foil 20 and the bump foil 30. The auxiliary foil 40 has the same shape as the top foil 20 and is a metal foil having a foil main body 41 and a pair of flanges 42, 42. The foil main body 41 extends in the circumferential direction along the cylindrical shape of the inner circumferential surface 11a of the insertion hole 11.

[0041] 6A, three foils, namely, the auxiliary foil 40, the bump foil 30, and the top foil 20, are stacked in this order and placed on, for example, a flat stage (not shown). Next, the pseudo shaft 102 is placed near the center of the top foil 20. The pseudo shaft 102 is a cylindrical member having a diameter equal to or slightly smaller than that of the shaft 2.

[0042] 6B , with the pseudo shaft 102 pressed against the top foil 20, the curved portion 21 a of the top foil 20 is temporarily spread to align the flanges 22, 32, and 42 of each foil in a substantially straight line. While maintaining this state, the pins 50 of the assembly robot are operated to insert the pins 50 into the flanges 22, 32, and 42 of each foil.

[0043] Next, as shown in Fig. 6C, the pin 50 inserted into the flanges 22, 32, 42 is moved to position P corresponding to the groove 12. This operation lifts up the flanges 22, 32, 42, and wraps the foil bodies 21, 31, 41 around the outer circumferential surface 102a of the pseudo shaft 102 (see Fig. 6D). When the foil bodies 21, 31 are wrapped around the outer circumferential surface 102a of the pseudo shaft 102, the auxiliary foil 40 supports the foil body 31 of the bump foil 30 from the radially outer side. This prevents the wavy foil body 31 from being deformed into an unexpected shape.

[0044] 6D, the pseudo shaft 102 is moved so that its central axis is aligned with the central axis of the bearing housing 10. At this time, the phase (angle) of the pseudo shaft 102 around its central axis is adjusted so that the flanges 22, 32, 42 overlap with the internal space of the groove 12 (see FIG. 2) of the bearing housing 10 when viewed from the axial direction.

[0045] Next, as shown in Fig. 6E, the pseudo shaft 102 is inserted into the insertion hole 11. Thereafter, the pin 50 is removed from the flanges 22, 32, 42 while preventing all of the foils 20, 30, 40 from slipping out of the insertion hole 11 (see Fig. 6F).

[0046] When the pin 50 is removed from the flanges 22, 32, 42, the foil bodies 21, 31, 41 loosen their grip on the pseudo shaft 102, and the foil bodies 21, 31, 41 expand toward the inner circumferential surface 11a of the insertion hole 11 due to their restoring force. Then, from this state, only the pseudo shaft 102 is removed from the insertion hole 11 (see FIG. 6F). The above series of steps completes the installation of the foils 20, 30, 40 into the insertion hole 11.

[0047] When the top foil 20 is attached to the insertion hole 11, the foil body 21 forms a bearing surface for the shaft 2. The foil body 21 is also discontinued between the pair of flanges 22. Therefore, the portion far from the flange 22 receives equal bending stress from both sides in the circumferential direction, while the bending stress received from the flange 22 side decreases as the portion approaches the flange 22. As a result, the cross section of the foil body 21 (bearing surface) is non-circular, i.e., teardrop-shaped, and the bearing 5 is configured as a non-circular journal bearing.

[0048] Next, the operation of the bearing 5 will be described.

[0049] When the shaft 2 is stationary, the shaft 2 is in contact with the top foil 20 (foil body 21) and is elastically supported by the bump foil 30 via the top foil 20. When the shaft 2 is rotating at a relatively slow speed, such as during startup, the shaft 2 rotates while remaining in contact with part of the top foil 20. However, except for the part of the top foil 20 that is in contact with the shaft 2, the fluid between the shaft 2 and the top foil 20 begins to flow in the same direction as the rotation of the shaft 2, forming a fluid film (hereinafter referred to as a fluid film).

[0050] Until the rotation speed of the shaft 2 reaches a certain value, the shaft 2 comes into contact with the top foil 20, blocking the flow of fluid in the circumferential direction. On the other hand, while the shaft 2 is rotating, the flow of fluid toward the contact point between the shaft 2 and the top foil 20 continues. The internal pressure of the fluid film increases as the rotation speed of the shaft 2 increases. When the internal pressure exceeds a certain value due to the increase in rotation speed, the shaft 2 separates from the top foil 20 (floats). In other words, the shaft 2 is supported by the bearing 5 without contact via the fluid film.

[0051] The internal pressure of this fluid film acts on the top foil 20 and presses the individual bump portions 33 of the bump foil 30 that are in contact with the top foil 20. At this time, the bump portions 33 are pushed outward in both circumferential directions. That is, the bump foil 30 elastically supports the top foil 20 due to the elastic deformation of the bump portions 33, and allows deformation and displacement of the top foil 20. In other words, the foil bodies 21, 31, 41 support the shaft 2 between the shaft 2 and the inner circumferential surface 11a of the insertion hole 11.

[0052] In this embodiment, the foil body 21 of the top foil 20 forms a bearing surface with a teardrop-shaped cross section that is close to a perfect circle. In other words, the bearing 5 according to this embodiment is a non-circular journal bearing, which generates anisotropy in the spring characteristics of the fluid film. As a result, compared to a circular journal bearing, it is possible to suppress phenomena such as oil whip and stabilize the position of the rotating shaft.

[0053] However, because the bearing surface forms a non-circular cross section, there is a certain frequency (duration) at which the shaft 2 comes into contact with the foil body 21. If this frequency (duration) increases, the foil wear will be more likely to progress, shortening the life (usable period) of the bearing.

[0054] Therefore, in the foil body 21 according to this embodiment, the curved portions 21 a are provided on both sides of the intermediate portion 21 b in the circumferential direction. Furthermore, when the top foil 20 is in an unloaded state, the radius of curvature of the curved portions 21 a is smaller than the radius of curvature of the intermediate portion 21 b. In other words, the curvature of the curved portions 21 a in an unloaded state is larger than the curvature of the intermediate portion 21 b in the same state.

[0055] When the foil body 21 having the curved portion 21 a is rolled into a cylindrical shape, the curved portion 21 a of the foil body 21 curves with a larger curvature (smaller radius of curvature) than when the foil body 21 does not have the curved portion 21 a. Therefore, while maintaining the teardrop-shaped cross-section, the cross-sectional shape can be made closer to a perfect circle than when the foil body 21 does not have the curved portion 21 a. In other words, by maintaining the teardrop-shaped cross-sectional shape, it is possible to suppress instability in the position of the shaft 2 during high-speed rotation, while by making the cross-sectional shape closer to a perfect circle, it is possible to reduce friction between the shaft 2 and the top foil 20 during low-speed rotation. In other words, according to this embodiment, it is possible to provide a foil for a radial foil bearing that can maintain bearing performance while extending the bearing's lifespan.

[0056] Next, some analytical results regarding the shape of the foil according to this embodiment will be described. Specifically, the results of numerical analysis that calculated the local contact force and starting torque for the bearing housing 10 having top foils of various shapes will be described.

[0057] 7 is a schematic diagram of a radial foil bearing 5A assumed in the numerical analysis. In this numerical analysis, a radial foil bearing 5A having a top foil X with various curved shapes as an evaluation target is assumed, and the local contact force and starting torque are calculated. The top foil X is the top foil shown in FIGS. 8A, 9A, and 10A.

[0058] As shown in Figure 7, the top foil X, together with the bump foil 30, is rolled into a cylindrical shape around the shaft 2 and placed in the insertion hole 11. In this analysis, it is assumed that the top foil X is rolled symmetrically with respect to a symmetry plane 8. The symmetry plane 8 is located, for example, on the center line of the foil body X1, which extends in the second direction B. The symmetry plane 8 and the outer peripheral surface 2a of the shaft 2 intersect at two points, forming intersection lines C1 and C2. As will be described later, the position of the intersection line C1 is treated as the origin of coordinates on the outer peripheral surface 2a along the circumferential direction.

[0059] When the shaft 2 is stopped from rotating, the shaft 2 comes into contact with the top foil X. Meanwhile, the rolled top foil X forms a teardrop-shaped cross section. Therefore, the contact force (pressure) that the outer circumferential surface 2a of the shaft 2 receives from the top foil X varies along the circumferential direction. The local contact force is a value obtained by integrating the above-mentioned contact force over the length of one bump portion 33. The starting torque is the torque required to rotate the shaft 2.

[0060] In the following numerical analysis, the above-mentioned local contact force and starting torque are calculated at each position in the circumferential direction. In this analysis, the diameter of the shaft 2 is constant, and the elastic modulus, thickness, and length along the first direction A of the material of the top foil X are all the same. The length of the top foil X is set to a value that covers approximately 98% of the outer circumference of the shaft 2. Meanwhile, the bump foil 30 is assumed to be the same regardless of the shape of the top foil X. Note that the dimensions of each component assumed in the analysis do not limit the dimensions of each component according to this embodiment.

[0061] (Analysis Result 1) First, the results of the first numerical analysis will be described. The first numerical analysis evaluated top foils with different positions or distributions of curved portions. FIG. 8A is a side view of the top foil assumed in the first numerical analysis. The top foil A1 corresponds to the top foil 20 according to this embodiment. Meanwhile, the top foils B1, B2, and B3 are top foils serving as comparative examples.

[0062] The top foil A1 has a foil body 21 according to this embodiment. The foil body 21 has a pair of curved portions 21 a and an intermediate portion 21 b located between the pair of curved portions 21 a. In this example, the intermediate portion 21 b is formed in a planar shape in its entirety in the no-load state.

[0063] On the other hand, the top foil B1 has a foil body 51. The foil body 51 has a pair of flat plate portions 51 a and a curved portion 51 b located between the pair of flat plate portions 51 a. The curved portion 51 b extends while curving from one of the pair of flat plate portions 51 a to the other. Therefore, the cross-sectional shape of the foil body 21 is approximately V-shaped.

[0064] The radius of curvature of the curved portion 21 a and the curved portion 51 b is equal to the diameter of the shaft 2 .

[0065] The top foil B2 has a foil body 61. The foil body 61 is curved as a whole in the no-load state, and its radius of curvature is twice the diameter of the shaft 2. Conversely, the foil body 71 of the top foil B3 is formed as a whole in a flat shape in the no-load state. All of the top foils A1, B1, B2, and B3 have flanges 22 of the same shape.

[0066] Figure 8B is a graph showing the results of numerical analysis when each of the top foils A1, B1, B2, and B3 shown in Figure 8A is used. The vertical axis of the graph shown in Figure 8B represents the local contact force described above. The horizontal axis represents coordinates on the outer peripheral surface 2a of the shaft 2 along the circumferential direction (hereinafter referred to as circumferential coordinates), with the position of the intersection line C1 shown in Figure 7 as the origin. The origin of the horizontal axis corresponds to the position of the intersection line C1 shown in Figure 7, and a negative sign attached to the circumferential coordinate indicates that the coordinate is located upstream of the position of the intersection line C1 in the rotation direction RD of the shaft 2.

[0067] The circumferential coordinates are normalized by the length from the position of the intersection line C1, which is the origin, to the position of the coordinate, which is half the length of the circumference of the outer peripheral surface 2a. For example, the coordinate of -1 indicates a position that is half the circumference of the outer peripheral surface 2a away from the origin (i.e., the position of the intersection line C2).

[0068] Although all of the top foils A1, B1, B2, and B3 are rolled into a cylindrical shape, the ends of the foil bodies 21, 51, 61, and 71 are not connected to each other. Therefore, the curvature of the foil bodies 21, 51, 61, and 71 decreases toward the ends of the foil bodies. Due to this tendency, the contact force between the shaft 2 and the foil bodies 21, 51, 61, and 71 tends to increase near the ends of the foil bodies. As an example of the results, FIG. 8B shows that the local contact force of the top foils B1 to B3 increases at circumferential coordinates from -0.95 to -0.8. However, the foil body 61 of the top foil B2 is pre-curved overall. Therefore, the local contact force near the ends of the foil body 61 is relaxed and slightly lower than that of the top foils B1 and B3.

[0069] On the other hand, in the range where the local contact force of the top foils B1 to B3 increases, the local contact force of the top foil A1 decreases. The curved portion 21a of the top foil A1 is formed in the circumferential coordinate range of -1 to -0.72, which range includes the range where the local contact force of the top foils B1 to B3 increases. In other words, it can be seen that providing the curved portion 21a at the end of the foil main body reduces the local contact force near the end.

[0070] Furthermore, the starting torque was the smallest when the top foil A1 was used. However, no significant difference was observed in the displacement of the shaft 2 when the same load was applied to any of the top foils. From the above analysis results, it can be seen that the shape of the top foil A1 keeps the displacement of the shaft 2 at the same level as the other shapes, while minimizing both the local contact force and the starting torque.

[0071] (Analysis Result 2) Next, the results of the second numerical analysis will be described. In the second numerical analysis, the influence of changes in the occupancy rate of the curved portions 21a in the foil body 21 according to this embodiment on the local contact force and starting torque was evaluated. The occupancy rate of the curved portions 21a is the proportion of the region from the end of the foil body 21 to the plane of symmetry 8 (see FIG. 7 ) that is occupied by the curved portions 21a. The length of this region along the first direction A is approximately equal to half the total length of the foil body 21.

[0072] 9A is a side view of the top foils A2 to A6 according to the present embodiment and the top foil B4 according to the comparative example. The occupancy rates of the curved portions 21a are set to 46%, 37%, 28%, 22%, and 14%, respectively, from the top foil A2 to the top foil A6. In all of the top foils A2 to A6, the radius of curvature of the curved portions 21a is the same in the no-load state, and the foil main body 21 has the same length. Furthermore, the entire intermediate portion 21b is formed flat in the no-load state. On the other hand, the occupancy rate of the curved portions 21a in the top foil B4 is 0%. That is, the foil main body 81 of the top foil B4 does not have the curved portions 21a and is formed flat in its entirety.

[0073] FIG. 9B is a graph showing the analysis results when using each of the top foils A2 to A6 and B4 shown in FIG. 9A. The horizontal axis represents the occupancy rate of the curved portion 21a. The left vertical axis represents the local contact force, with the calculated values ​​indicated by white circles. Meanwhile, the right vertical axis represents the starting torque, with the calculated values ​​indicated by black circles. Note that the values ​​in parentheses on the horizontal axis represent the occupancy rate of the intermediate portion 21b. The occupancy rate of the intermediate portion 21b is the proportion of the intermediate portion 21b from the symmetry plane 8 (see FIG. 7) toward the foil body 21. The occupancy rate for each of the top foils A2 to A6 and B4 shown in FIG. 9A is indicated by arrows on the horizontal axis.

[0074] As shown in Figure 9B, both the local contact force and the starting torque are greatest when the top foil B4 is used, i.e., when the occupancy of the curved portion 21a is 0% (the occupancy of the middle portion 21b is 100%), and decrease when the occupancy is greater than 0% and less than or equal to 50%. This indicates that the presence of the curved portion 21a reduces the local contact force and the starting torque. Furthermore, the local contact force is minimized when the occupancy of the curved portion 21a is 37% (the occupancy of the middle portion 21b is 63%), and the starting torque is minimized when the occupancy of the curved portion 21a is approximately 28% (the occupancy of the middle portion 21b is 72%). Considering the width of the bump portion 33 of the bump foil 30 assumed in this analysis, the length of the curved portion 21a at which the local contact force and the starting torque are minimized corresponds to the length of three to four bump portions 33 from the end of the foil body 31.

[0075] The occupancy rate of the curved portion 21a at which the local contact force and starting torque are minimum varies depending on the diameter of the shaft 2, the diameter of the insertion hole 11, the radius of curvature of the curved portion 21a, the radius of curvature of the intermediate portion 21b, the elastic modulus and thickness of the foil material, and the shape and dimensions of the bump portion, etc. However, it can be roughly estimated that the local contact force and starting torque are minimum when the occupancy rate of the curved portion 21a is set to a value between 20% and 50%.

[0076] That is, in the top foil including the intermediate portion 21b that is entirely formed in a planar shape in the no-load state, by setting the distance between the pair of curved portions 21a to at least half the total length of the foil body 31 along the first direction A, it is possible to reduce the local contact force and starting torque compared to a foil body that is entirely formed in a planar shape, at least in the no-load state. Furthermore, by optimizing the curvature of the curved portions 21a, the elastic modulus and thickness of the foil material, etc., it is possible to minimize the local contact force and starting torque.

[0077] (Analysis Result 3) Next, the results of the third numerical analysis will be described. In the third numerical analysis, the influence of changes in the radius of curvature of the curved portion 21a on the local contact force and the starting torque was evaluated. Fig. 10A is a side view of the top foils A7 to A10 according to this embodiment and the top foil B4 as a comparative example.

[0078] FIG. 10A is a side view of the top foils A7 to A10 according to this embodiment and the top foil B4 according to the comparative example. The radii of curvature of the curved portions 21a of the top foils A7 to A10 are set to 0.52, 0.76, 1, and 1.48 times the diameter of the shaft 2, respectively, from the top foil A7 to the top foil A10. However, the overall length of the foil body 21 and the occupancy rate of the curved portions 21a of the top foils A7 to A10 are set to the same values. In this analysis, the occupancy rate of the curved portions 21a is set to 28%, which is the value that resulted in the minimum starting torque in the second numerical analysis. Meanwhile, the top foil B4 does not have a curved portion 21a. In other words, if the top foil B4 is considered to have a curved portion 21a, its radius of curvature is infinite.

[0079] Figure 10B is a graph showing the analysis results when the top foils A7 to A10 and B4 shown in Figure 10A were used. The horizontal axis represents the value obtained by dividing the diameter D of the shaft 2 by the curvature radius R of the curved portion 21a (hereinafter referred to as the D / R value). The left vertical axis represents the local contact force, and its calculated values ​​are indicated by white circles. Meanwhile, the right vertical axis represents the starting torque, and its calculated values ​​are indicated by black circles.

[0080] The top foil for which the D / R value is 0 has an infinite radius of curvature R, i.e., the top foil B4 does not have the curved portion 21a. As shown in Figure 10B, the local contact force is reduced by the presence of the curved portion 21a. It can also be seen that this value is approximately constant regardless of the value of the radius of curvature R (at least within the range of D / R values ​​of the top foils A7 to A10).

[0081] Like the local contact force, the starting torque is also reduced by the presence of the curved portion 21a. The decreasing trend of the starting torque becomes gentler when the D / R value reaches approximately 1, and the starting torque becomes almost constant when the D / R value exceeds 1. However, the D / R value at which the decreasing trend becomes gentler is not uniquely determined and may vary depending on the elastic modulus and thickness of the foil material, etc.

[0082] The second and third numerical analyses show that the local contact force and starting torque can be reduced by providing the curved portion 21a on the foil body 31 and further by appropriately setting the occupancy rate of the curved portion 21a. However, even if the occupancy rate is set, the radius of curvature of the curved portion 21a does not need to be fixed to a single value corresponding to the occupancy rate, and the degree of freedom in setting the radius of curvature is high. Therefore, the radius of curvature can be set appropriately taking into account factors such as ease of assembly of the bearing.

[0083] The top foil according to this embodiment has the curved portion 21a described above, which contributes to reducing the local contact force and starting torque. Considering this, the foil according to this embodiment is not limited to being used as a top foil, but can also be used as a bump foil, auxiliary foil, or other foil that is overlapped with the top foil. That is, the radial foil bearing according to this embodiment includes foils (foil bodies) that are overlapped with each other, and the foil (foil body) according to this embodiment is used as at least one of the foils.

[0084] 11 is a side view of a bump foil 30 to which the foil according to this embodiment is applied. As shown in FIG. 11, the foil body 31 of the bump foil 30 includes a pair of curved portions 31 a, 31 a and a middle portion 31 b located between the pair of curved portions 31 a, 31 a.

[0085] As described above, the foil body 31 of the bump foil 30 is formed in a wave shape that extends in the first direction A (circumferential direction) while meandering in the surface direction (radial direction) of the foil body 31. However, the curved portion 31 a is curved in a direction that causes the bump foil 30 to be rounded even in an unloaded state due to plastic deformation caused by machining such as bending.

[0086] The intermediate portion 31b extends with a larger radius of curvature than the curved portion 31a. For example, as shown in Fig. 11, the intermediate portion 21b may be expanded in a flat state in an unloaded state (i.e., the radius of curvature is infinite). The radius of curvature of the curved portion 31a can be defined by a line connecting multiple positions that are locally equivalent (e.g., multiple connecting portions 34, apexes or bases of multiple bump portions 33, etc.).

[0087] As the above three analysis results show, when curved portions 21a are provided at both ends of the middle portion 21b in the foil body 21 of the top foil 20, the local contact force and starting torque can be reduced. When a similar curved shape is provided to the bump foil 30, the contact force between the bump foil 30 and the top foil 20 is expected to be alleviated. In other words, the bump foil 30 provided with the curved portions 21a contributes to reducing the local contact force and starting torque.

[0088] The present disclosure is not limited to the above-described embodiments, but is defined by the claims and includes all modifications within the meaning and scope of the claims. For example, the dimensions assumed in the above numerical analysis are merely illustrative and may be modified as appropriate as long as the effects described above are obtained.

Claims

1. A foil for a radial foil bearing, comprising a foil body having a length in a first direction that allows it to be rolled into a cylindrical shape, the foil body including a pair of curved portions spaced apart from each other in the first direction, and an intermediate portion located between the pair of curved portions and extending from one of the pair of curved portions to the other, and in an unloaded state where the foil body is not receiving an external force for rolling, the pair of curved portions are curved with a radius of curvature that is smaller than the radius of curvature of the intermediate portion.

2. A foil according to claim 1, wherein the intermediate portion is formed to be entirely flat in the no-load state, and the distance between the pair of curved portions is set to at least half the total length of the foil body along the first direction.

3. The foil according to claim 1, wherein in the unloaded state, at least a part of the intermediate portion is formed in a planar shape.

4. A foil according to any one of claims 1 to 3, comprising a pair of flanges provided at both ends of the foil body in the first direction and folded back in a direction opposite to the direction in which the foil body is rolled.

5. A foil according to any one of claims 1 to 3, wherein the foil body is formed as a top foil or an intermediate foil.

6. A foil according to any one of claims 1 to 3, wherein the foil body is formed as a bump foil.

7. A radial foil bearing comprising a foil according to any one of claims 1 to 6.

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