Radial foil bearing

Abstract

Individual foils 12 of a radial foil bearing 10 include a top foil part 12a having a bearing surface S facing a rotary member 6, and an under-foil part 12c provided on the top foil part 12a and arranged overlapping the side of an adjacent foil opposite to the bearing surface S of the top foil part 12a. The under-foil part 12c has a plurality of opening parts 13a.

Description

Radial Foil Bearings

[0001] The present invention relates to a radial foil bearing.

[0002] Foil bearings have a bearing surface made of a thin, flexible foil with low bending stiffness, and support a load while allowing the bearing surface to deflect. When the shaft rotates, a fluid film (e.g., an air film) forms between the shaft and the foil, and this fluid film supports the shaft without contact.

[0003] At this time, the foil flexes, automatically forming an appropriate bearing gap according to the operating conditions, such as the shaft rotation speed, load, and ambient temperature, thereby providing excellent stability and enabling support for a high-speed rotating shaft.

[0004] For example, Patent Document 1 discloses a radial foil bearing including a foil holder and a plurality of foils attached to the foil holder in a circumferentially aligned arrangement. In this radial foil bearing, each foil includes a top foil portion having a bearing surface, an extension portion provided on one side of the top foil portion, and an under foil portion provided on the other side of the top foil portion (see claim 1 of the same document). The under foil portion is disposed between the top foil portion of another foil and the foil holder to support the top foil portion.

[0005] Figure 14 corresponds to Figure 4 of the same document. As shown in Figure 14, the under foil portion UF has a cutout portion N configured in a concave arc shape. The top foil portion TF of another foil FL supported by the under foil portion UF deforms in accordance with the shape of the cutout portion N during rotation of the shaft. This deformation causes a step to be formed in the top foil portion TF along the cutout portion N of the under foil portion UF. The fluid flowing along the top foil portion TF during rotation of the shaft is collected in the step, which makes it possible to increase the pressure acting on the bearing gap of the radial foil bearing (see paragraph 0032 of the same document).

[0006] JP 2017-96324 A

[0007] Figure 15 shows the state of the radial foil bearing when the axis AX is not rotating. Also, Figure 15 shows a cross section of the foil FL along the line XV-XV in Figure 14. The foil FL is housed between the axis AX and the foil holder FF.

[0008] 15 , since the cutout portion N is formed in the underfoil portion UF, the central portion of the top foil portion TF in the axial direction AD is not supported by the underfoil portion UF. The underfoil portion UF supports the top foil portion TF by a first protrusion UFa located at one end in the axial direction AD and a second protrusion UFb located at the other end in the axial direction AD.

[0009] 16 shows the state of the radial foil bearing when the shaft AX is rotating. In this case, the top foil portion TF is deformed into a concave shape according to the shape of the cutout portion N. If the deformation of the top foil portion TF becomes excessive, the ends TFa and TFb in the axial direction AD may approach the shaft AX and come into contact with the outer circumferential surface of the shaft AX. Because the radial foil bearing supports the shaft AX rotating at high speed, if the top foil portion TF comes into contact with the shaft AX, there is a risk that the shaft AX may become seized, leading to damage to the radial foil bearing.

[0010] The present invention has been made in view of the above circumstances, and has as its technical object to prevent the top foil portion from coming into contact with the rotating member without causing a drop in pressure.

[0011] The present invention is intended to solve the above-mentioned problems, and provides a radial foil bearing comprising a plurality of foils arranged in a line in the rotational direction of a rotating member, and a foil holder that houses the foils, wherein the foils have a top foil portion having a bearing surface facing the rotating member, and an under foil portion that is provided upstream of the top foil portion in the rotational direction and is arranged overlapping the opposite side of the bearing surface of the top foil portion of an adjacent other foil, and the under foil portion has a plurality of openings.

[0012] According to this configuration, by forming multiple openings in the underfoil portion, it is possible to reduce the rigidity of the underfoil portion. This allows the underfoil portion to deform so as to follow the deformation of the top foil portion. By allowing the underfoil portion to deform in the same way as the top foil portion, it is possible to suppress excessive deformation of the top foil portion without causing a decrease in pressure, and to prevent the top foil portion from coming into contact with the rotating member.

[0013] In the above-mentioned radial foil bearing, the opening may include a first opening having a large opening area and a second opening having a small opening area, and the first opening may be formed in a central portion of the underfoil portion in the direction of the rotation axis of the rotating member.

[0014] According to this configuration, by forming the first opening with a large opening area in the center of the underfoil portion, it is possible to reduce the rigidity of this center portion. This makes it easier for the center of the underfoil portion to deform in accordance with the deformation of the top foil portion. Therefore, the portion of the top foil portion corresponding to the center of the underfoil portion is easier to deform, and by effectively guiding the fluid to this portion, the radial foil bearing can increase the pressure that supports the rotating member.

[0015] In the above-mentioned radial foil bearing, the opening may include a first opening having a large opening area and a second opening having a small opening area, and the first opening may be formed on the upstream side in the rotation direction of the rotating member, and the second opening may be formed on the downstream side in the rotation direction of the rotating member.

[0016] According to this configuration, by forming a first opening with a large opening area on the upstream side in the rotation direction and forming a second opening with a small opening area on the downstream side in the rotation direction, the underfoil portion is configured to have low rigidity in the upstream region and high rigidity in the downstream region.

[0017] Therefore, the top foil portion supported by the under foil portion is more easily deformed on the upstream side and less easily deformed on the downstream side. As a result, the bearing gap formed between the top foil portion and the rotating member is larger on the upstream side and smaller on the downstream side, forming a wedge-like shape. By configuring the bearing gap in this wedge shape, the fluid pressure in the bearing gap can be increased, allowing the radial foil bearing to stably support the rotating member.

[0018] In the above-described radial foil bearing, the foil holder may have an inner circumferential surface facing the foil, and the under foil portion may have a protrusion protruding toward the inner circumferential surface of the foil holder.

[0019] According to this configuration, the protrusions of the underfoil portion can be brought into contact with the inner circumferential surface of the foil holder while the rotating member is rotating. By bringing the protrusions into contact with the foil holder and causing slight sliding, the frictional energy can be used to effectively damp vibrations of the rotating member.

[0020] According to the present invention, it is possible to prevent the top foil portion from coming into contact with the rotating member without causing a drop in pressure.

[0021] 1 is a diagram conceptually showing the configuration of a gas turbine. FIG. 1 is a cross-sectional view showing a rotor support structure in a gas turbine. FIG. 2 is a cross-sectional view of a radial foil bearing incorporated in the rotor support structure. FIG. 3 is a plan view showing an example of a foil. FIG. 4 is a plan view showing a developed view of a plurality of connected foils. FIG. 5 is a cross-sectional view showing a portion of a foil. FIG. 6 is a plan view showing another example of a foil. FIG. 7 is a cross-sectional view of a radial foil bearing supporting a rotating shaft. FIG. 8 is a cross-sectional view of a radial foil bearing. FIG. 9 is a plan view of a foil according to a comparative example. FIG. 10 is a graph showing the results of a performance test of a radial foil bearing. FIG. 11 is a graph showing the results of a performance test of a radial foil bearing. FIG. 12 is a plan view showing the developed connected state of conventional foils. FIG. 13 is a cross-sectional view of a conventional radial foil bearing when the shaft is not rotating. FIG. 14 is a cross-sectional view of a conventional radial foil bearing when the shaft is rotating.

[0022] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0023] 1 conceptually illustrates the configuration of a gas turbine, a type of turbomachinery. This gas turbine mainly comprises a turbine 1 and a compressor 2, each of which has a blade row, a generator 3, a combustor 4, and a regenerator 5. A common shaft 6 extending horizontally is provided for the turbine 1, the compressor 2, and the generator 3, and this shaft 6, the turbine 1, and the compressor 2 form a rotor that can rotate integrally with each other.

[0024] Air drawn in through an intake port 7 is compressed by a compressor 2, heated in a regenerator 5, and then sent to a combustor 4. This compressed air is mixed with fuel and burned, and the high-temperature, high-pressure gas rotates a turbine 1. The rotational force of the turbine 1 is transmitted to a generator 3 via a shaft 6, and the rotation of the generator 3 generates electricity, which is then output via an inverter 8.

[0025] The gas after rotating the turbine 1 is at a relatively high temperature, so this gas is sent to the regenerator 5 and heat exchanged with the compressed air before combustion, thereby reusing the heat of the gas after combustion. After heat exchange in the regenerator 5, the gas passes through an exhaust heat recovery device 9 and is then discharged as exhaust gas.

[0026] 2 shows the rotor support structure, in particular the support structure for the shaft 6 between the turbine 1 and the compressor 2. This region is adjacent to the turbine 1, which is rotated by high-temperature, high-pressure gas, so an air dynamic bearing, in particular a foil bearing, is preferably used here.

[0027] Specifically, the shaft 6 is supported in the radial direction by the radial foil bearing 10, and a pair of thrust foil bearings 20 support a thrust collar 6a provided on the shaft 6 in both thrust directions.

[0028] As shown in Figure 3, the radial foil bearing 10 supports the shaft 6 in the radial direction by a fluid film generated in a radial bearing gap G between the bearing and the shaft 6, which serves as a rotating member inserted into the inner periphery. The radial foil bearing 10 of this embodiment is an air dynamic bearing that uses air as a pressure-generating fluid. The radial foil bearing 10 includes a cylindrical foil holder 11 and a plurality of foils 12 (three in the illustrated example) that are arranged along the rotation direction RD of the shaft 6 and attached to the inner circumferential surface 11a of the foil holder 11. In the following description, the downstream side in the rotation direction RD of the shaft 6 (see Figure 3) may be simply referred to as the "downstream side," and the upstream side in the rotation direction RD may be simply referred to as the "upstream side."

[0029] The foil holder 11 is made of metal or resin. Examples of metals that form the foil holder 11 include sintered metal and ingot material (e.g., steel). The foil holder 11 has a cylindrical inner circumferential surface 11a and an outer circumferential surface 11b. Axial grooves 11c are formed on the inner circumferential surface 11a of the foil holder 11 at multiple locations (three locations in the illustrated example) spaced apart in the circumferential direction (the rotation direction RD of the shaft 6). Both axial ends of each axial groove 11c open to the end face of the foil holder 11.

[0030] The foils 12 are connected to each other to form a cylindrical shape. Each foil 12 is accommodated inside the inner circumferential surface 11a of the foil holder 11 so as to face the inner circumferential surface 11a. Each foil 12 is made of a thin metal film.

[0031] As shown in Figure 4, each foil 12 has a top foil portion 12a, an insertion portion 12b provided downstream of the top foil portion 12a (left side of Figure 4), and an under foil portion 12c provided upstream of the top foil portion 12a (right side of Figure 4).

[0032] The inner diameter side surface of the top foil portion 12a functions as a bearing surface S that faces the outer circumferential surface of the shaft 6 in the radial direction (see FIG. 3). The insertion portions 12b extend downstream (to the left in FIG. 4) from the top foil portion 12a and are formed at both ends of the top foil portion 12a in the axial direction (rotation axis direction) AD in the illustrated example. An insertion port 12d is provided at the boundary between the top foil portion 12a and the under foil portion 12c. The insertion portion 12b of each foil 12 is inserted from the inner diameter side into the insertion port 12d of the foil 12 adjacent to it on the downstream side (see FIG. 5) and is further inserted into the axial groove 11c of the foil holder 11 (see FIG. 3).

[0033] The under foil portion 12c of each foil 12 is disposed so as to overlap the outer diameter side (opposite the bearing surface S) of the top foil portion 12a of the foil 12 adjacent to the upstream side. Specifically, the under foil portion 12c of each foil 12 is disposed between the top foil portion 12a of the foil 12 adjacent to the upstream side and the inner circumferential surface 11a of the foil holder 11 (see FIG. 3). As a result, the top foil portion 12a of each foil 12 is supported from the outer diameter side by the under foil portion 12c of the adjacent foil 12.

[0034] As shown in FIG. 4 , the underfoil portion 12c of each foil 12 has multiple openings 13a, 13b, multiple protrusions 14a, 14b, and partitions 15a-15c that separate the multiple openings 13a, 13b and the multiple protrusions 14a, 14b. The openings 13a, 13b, the protrusions 14a, 14b, and the partitions 15a-15c are formed by blanking the underfoil portion 12c. Examples of blanking methods include press processing, laser processing, and etching, but etching is most preferred because it suppresses deformation of the underfoil portion 12c during processing. The multiple openings 13a, 13b of the underfoil portion 12c are designed to deform in response to the deformation of the top foil portion 12a that occurs with the rotation of the shaft 6, which serves as a rotating member.

[0035] As shown in FIG. 4 , the openings 13a, 13b include a first opening 13a with a large opening area and a second opening 13b with a small opening area. The protrusions 14a, 14b include a first protrusion 14a formed adjacent to the first opening 13a and a second protrusion 14b formed adjacent to the second opening 13b and larger than the first protrusion 14a. The size relationship between the opening areas of the first opening 13a and the second opening 13b and the size relationship between the first protrusion 14a and the second protrusion 14b are not limited to this embodiment. The first opening 13a is formed to surround the first protrusion 14a. The second opening 13b is formed to surround the second protrusion 14b.

[0036] As shown in Fig. 4, the openings 13a, 13b and the protrusions 14a, 14b are arranged in multiple rows and multiple columns at regular intervals. Specifically, the openings 13a, 13b and the protrusions 14a, 14b are formed in three rows along the axial direction AD. That is, the underfoil portion 12c has a first row C1 of openings 13a, 13b and protrusions 14a, 14b that is located most upstream in the rotational direction RD, a second row C2 of openings 13a, 13b and protrusions 14a, 14b that is located downstream of the first row C1, and a third row C3 of openings 13b and protrusions 14b that is located downstream of the second row C2.

[0037] The first row C1 is composed of a plurality of first openings 13 a and second openings 13 b, but is not limited to this and may be composed of only the first openings 13 a. The third row C3 is composed of a plurality of second openings 13 b, but may be composed of an opening having an opening area different from that of the second openings 13 b in combination with the second openings 13 b.

[0038] The second row C2 is composed of a plurality of first openings 13a and a plurality of second openings 13b. In the second row C2, the first openings 13a are located in a center portion CA of the underfoil portion 12c in the axial direction AD. Furthermore, the first openings 13a in the second row C2 are located in a center portion of the underfoil portion 12c in the rotational direction RD. The second openings 13b in the second row C2 are located outward in the axial direction AD from the first openings 13a.

[0039] Of the multiple first protrusions 14a formed in the first row C1 and the second row C2, two first protrusions 14a adjacent to each other in the axial direction AD protrude in opposite directions in the rotational direction RD (circumferential direction). That is, of the two first protrusions 14a, one first protrusion 14a protrudes toward the upstream side in the rotational direction RD, and the other first protrusion 14a protrudes toward the downstream side in the rotational direction RD.

[0040] In the second row C2, the first protrusions 14a and the second protrusions 14b adjacent to each other in the axial direction AD protrude in opposite directions in the rotational direction RD (circumferential direction). Similarly, among the plurality of second protrusions 14b formed in the second row C2 and the third row C3, two second protrusions 14b adjacent to each other in the axial direction AD protrude in opposite directions in the rotational direction RD (circumferential direction).

[0041] As shown in FIG. 4, the partitions 15a to 15c include a first partition 15a, a second partition 15b, and a third partition 15c.

[0042] The first partition portion 15a is a portion that separates two adjacent openings 13a, 13b in the axial direction AD. That is, the first partition portion 15a is formed so as to separate adjacent first openings 13a among the plurality of first openings 13a formed in the first row C1 and the second row C2. The first partition portion 15a is also formed so as to separate adjacent first openings 13a and second openings 13b in the second row C2. Furthermore, the first partition portion 15a is formed so as to separate adjacent second openings 13b in the third row C3.

[0043] The second partition portion 15b separates the first openings 13a formed in the first row C1 from the first openings 13a or the second openings 13b formed in the second row C2. The first openings 13a formed in the first row C1 and the first openings 13a or the second openings 13b formed in the second row C2 are adjacent to each other in the rotational direction RD via the second partition portion 15b.

[0044] The second partitions 15b also function as connecting portions that connect the first protrusions 14a formed in the first row C1 to the first protrusions 14a or the second protrusions 14b formed in the second row C2. The second partitions 15b also function as connecting portions that connect the first partitions 15a formed in the first row C1 to the first partitions 15a formed in the second row C2.

[0045] The third partition portion 15c separates the first openings 13a or the second openings 13b formed in the second row C2 from the second openings 13b formed in the third row C3. The first openings 13a or the second openings 13b formed in the second row C2 and the second openings 13b formed in the third row C3 are adjacent to each other in the rotational direction RD via the third partition portion 15c.

[0046] The third partition 15c also functions as a connecting portion that connects the first protrusion 14a or the second protrusion 14b formed in the second row C2 to the second protrusion 14b formed in the third row C3. The third partition 15c also functions as a connecting portion that connects the first partition 15a formed in the second row C2 to the first partition 15a formed in the third row C3.

[0047] Each of the protrusions 14a, 14b is configured to protrude outward from the underfoil portion 12c when the underfoil portion 12c is deformed into a curved shape. This will be described below with reference to Fig. 6. Fig. 6 shows a cross section of the pair of first protrusions 14a1, 14a1 shown in Fig. 4.

[0048] When the underfoil portion 12c is deformed into a curved shape, the first partition portion 15a adjacent to the first protrusion portion 14a1 is curved in accordance with the deformation of the underfoil portion 12c, as shown in Fig. 6. At this time, since the periphery of the first protrusion portion 14a1 is surrounded by the first opening 13a, the force that deforms the first partition portion 15a does not act on the first protrusion portion 14a1.

[0049] For this reason, the first protrusion 14a1 does not deform like the first partition portion 15a, and as a result, it protrudes outward more than the first partition portion 15a. That is, when the under foil portion 12c deforms as described above with the foil 12 attached to the foil holder 11, the first protrusion 14a1 protrudes toward the inner circumferential surface 11a of the foil holder 11. Similar to the first protrusion 14a1 (14a), the second protrusion 14b is also configured to protrude toward the inner circumferential surface 11a of the foil holder 11 when the under foil portion 12c is deformed.

[0050] 7 shows another example of the foil 12. In this example, the under foil portion 12c has first openings 13a and first protrusions 14a formed in a first row C1, second openings 13b and second protrusions 14b formed in a second row C2, and third openings 13c and third protrusions 14c formed in a third row C3.

[0051] In this example, the opening area of ​​the first opening 13 a is the largest, and the opening area of ​​the third opening 13 c is the smallest. The opening area of ​​the second opening 13 b is smaller than the opening area of ​​the first opening 13 a and larger than the opening area of ​​the third opening 13 c.

[0052] The size relationship between the protrusions 14a to 14c is such that the first protrusion 14a is the smallest and the third protrusion 14c is the largest. The second protrusion 14b is larger than the first protrusion 14a and smaller than the third protrusion 14c. The other configurations of this example are the same as those of the example shown in FIG.

[0053] Figure 8 shows another example of the foil 12. In this example, the underfoil portion 12c has multiple openings 13a with the same opening area. These openings 13a have the same configuration as the first openings 13a in the examples of Figures 4 and 7. The underfoil portion 12c also has multiple protrusions 14a of the same size. These protrusions 14a have the same configuration as the first protrusions 14a in the examples of Figures 4 and 7.

[0054] The following describes the operation of the radial foil bearing 10. When the shaft 6 rotates, a fluid film (air film) is formed in the radial bearing gap G between the bearing surface S of the foil 12 of the radial foil bearing 10 and the outer circumferential surface of the shaft 6.

[0055] At this time, the top foil portion 12a of each foil 12 rides up onto the under foil portion 12c of the adjacent foil 12 and curves. As shown in Figure 9, the under foil portion 12c deforms in accordance with the deformation of the top foil portion 12a that accompanies the rotation of the shaft 6. In other words, the top foil portion 12a and the under foil portion 12c curve in the same manner as one another while maintaining contact with each other. Note that Figure 9 shows the cross-sectional state of the foil 12 at the position indicated by the arrows IX-IX in Figure 5.

[0056] In this way, by deforming the under foil portion 12c so as to follow the deformation of the top foil portion 12a, excessive deformation of the top foil portion 12a can be suppressed, and the top foil portion 12a can be prevented from contacting the outer circumferential surface of the shaft 6. Furthermore, due to the above-described deformation, the end portion 12a1 of the top foil portion 12a in the axial direction AD can form a good bearing surface S. Therefore, the fluid pressure between the shaft 6 and the bearing surface S is not reduced.

[0057] Furthermore, in the foil 12 shown in Fig. 4, openings 13a and 13b with different opening areas are formed in the underfoil portion 12c. That is, the underfoil portion 12c has a first opening 13a with a large opening area on the upstream side in the rotation direction RD, and a second opening 13b with a small opening area on the downstream side of the first opening 13a. As a result, as shown in Fig. 10, the underfoil portion 12c has a structure in which the rigidity is low in a region A1 upstream in the rotation direction RD where the first opening 13a is formed, and the rigidity is high in a region A2 downstream in the rotation direction RD where the second opening 13b is formed.

[0058] With the above configuration, the bearing gap G (see FIG. 10 ) formed between the bearing surface S of the top foil portion 12 a and the outer circumferential surface of the shaft 6 forms a wedge shape that narrows from the region A1 of the first opening 13 a on the upstream side to the region A2 of the second opening 13 b on the downstream side. Therefore, as the shaft 6 rotates, air is forced into the narrow side of the wedge-shaped bearing gap G, increasing the pressure of the air film in the bearing gap G.

[0059] Furthermore, the top foil portion 12a of each foil 12 rides up on the under foil portion 12c of the adjacent foil 12, causing each foil 12 to curve. Due to the springiness caused by this curvature, each foil 12 deforms arbitrarily in response to operating conditions such as the load, the rotational speed of the shaft 6, and the ambient temperature, so the bearing gap G is automatically adjusted to an appropriate width in response to the operating conditions. Therefore, even under harsh conditions such as high temperature and high rotation speed, the bearing gap G can be maintained at an optimal width, enabling stable support of the shaft 6.

[0060] Furthermore, each foil 12 is not completely fixed to the foil holder 11, but is movable relative to the foil holder 11. Therefore, while the shaft 6 is rotating, minute sliding occurs between each foil 12 and the foil holder 11, and between adjacent foils 12. Frictional energy caused by this minute sliding can damp vibrations of the shaft 6.

[0061] In addition, the protrusions 14a, 14b formed on the underfoil portion 12c contact the inner circumferential surface 11a of the foil holder 11 while protruding toward the inner circumferential surface 11a. The contact with the inner circumferential surface 11a causes the protrusions 14a, 14b to elastically deform. The contact of the protrusions 14a, 14b with the inner circumferential surface 11a of the foil holder 11 can further improve the damping performance.

[0062] The present invention is not limited to the configuration of the above-described embodiment, nor is it limited to the above-described effects. The present invention can be modified in various ways without departing from the spirit of the present invention.

[0063] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0064] The inventors conducted a test to confirm the performance of the radial foil bearing according to the present invention. In this test, radial foil bearings using the foils according to Examples 1 to 3 and a radial foil bearing using the foil according to the comparative example were prepared.

[0065] The foil used in Example 1 was the one illustrated in Figure 4. The foil used in Example 2 was the one illustrated in Figure 7. The foil used in Example 3 was the one illustrated in Figure 8. The foil used in Comparative Example was the one illustrated in Figure 11. The foil FL in Comparative Example does not have an opening or a protrusion in the under foil portion UF formed on the upstream side of the top foil portion TF, but has a concave cutout portion N whose width in the axial direction AD narrows from the upstream side to the downstream side in the rotation direction RD. The foil in this Comparative Example is equivalent to the conventional foils shown in Figures 14 to 16.

[0066] In this test, the load capacity (pressure generated in the bearing gap) of the radial foil bearing was measured when the shaft was rotated while supported by the radial foil bearing. Specifically, the change in the load capacity (N) of the radial foil bearing when the rotation speed of the shaft was changed and the load capacity of the radial foil bearing when the shaft was rotated at a constant rotation speed (35 krpm) were measured. The test results are shown in Figures 12 and 13.

[0067] As shown in Figure 12, the load capacity of the radial foil bearings according to Examples 1 to 3 was higher than that of the radial foil bearing according to the comparative example, regardless of changes in rotation speed. Also, as shown in Figure 13, even when the shaft was rotated at a constant high speed, the load capacity of the radial foil bearings according to Examples 1 to 3 was higher than that of the radial foil bearing according to the comparative example.

[0068] This is thought to be because, as described above, the underfoil portion, which has multiple openings, deforms to follow the top foil portion, preventing excessive deformation of the top foil portion and increasing the area of ​​the bearing surface.

[0069] Furthermore, the radial foil bearings according to Examples 1 and 2 exhibited a higher load capacity than the radial foil bearing according to Example 3. This is thought to be due to the fact that, as described above, in the underfoil portion, the rigidity of the upstream region (A1) in the direction of rotation (RD) was reduced and the rigidity of the downstream region (A2) was increased (see FIG. 10 ), and the bearing gap was formed in a wedge shape.

[0070] Furthermore, when comparing the variations in load capacity, the variations in Example 1 and Comparative Example are smaller than those in Examples 2 and 3 (see FIG. 13 ). This is thought to be because, in Example 1, the first opening with a large opening area is formed in the center of the underfoil portion (see FIG. 4 ), which makes the center of the top foil portion more likely to deform, causing the fluid (air) to concentrate at the center of the bearing surface. Similarly, in the Comparative Example, the formation of a cutout portion in the underfoil portion is thought to be because the fluid is configured to concentrate at the center of the bearing surface of the top foil portion.

[0071] 6 Shaft (rotating member) 10 Radial foil bearing 11 Foil holder 11a Inner peripheral surface 12 Foil 12a Top foil portion 12c Under foil portion 13a First opening 13b Second opening 13c Third opening 14a First protrusion 14b Second protrusion 14c Third protrusion AD Axial direction (rotational axis direction) RD Rotational direction S Bearing surface

Claims

1. A radial foil bearing comprising a plurality of foils arranged in a line in the direction of rotation of a rotating member and a foil holder that houses the foils, wherein the foils have a top foil portion having a bearing surface that faces the rotating member, and an under foil portion that is provided upstream of the top foil portion in the direction of rotation and is arranged overlapping the other adjacent foil on the opposite side to the bearing surface of the top foil portion, and the under foil portion has a plurality of openings.

2. A radial foil bearing as set forth in claim 1, wherein the openings include a first opening having a large opening area and a second opening having a small opening area, and the first opening is formed in a central portion of the underfoil portion in the direction of the rotation axis of the rotating member.

3. A radial foil bearing as set forth in claim 1, wherein the openings include a first opening having a large opening area and a second opening having a small opening area, the first opening being formed on the upstream side in the rotation direction of the rotating member, and the second opening being formed on the downstream side in the rotation direction of the rotating member.

4. A radial foil bearing according to any one of claims 1 to 3, wherein the foil holder has an inner peripheral surface facing the foil, and the underfoil portion has a protrusion that protrudes toward the inner peripheral surface of the foil holder.

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