foil bearing

Abstract

To provide a foil bearing capable of increasing a load by forming floating gaps at three places near a tip end of a top foil portion, near an arc portion at an inner diameter end, and an arc portion of an outer diameter end.SOLUTION: A foil bearing is formed with a bearing surface by aligning a plurality of leaves 22 in a circumferential direction. The leaf 22 has a top foil portion 22a forming the bearing surface, and an inner diameter end and an outer diameter end of the leaf 22 are arc portions. By regulating a ratio between diameter dimensions D2 of an arc portion 22e at the inner dimeter end of the leaf and diameter dimensions D1 of an arc portion 22d at the outer dimeter end of the leaf 22, and a ratio between a center angle B of the arc portion at the inner diameter end and a center angle A of the arc portion at the outer diameter portion, floating gaps are formed at three places near a tip end of the top foil portion 22a, near the arc portion 22e at the inner diameter end, and near the arc portion 22d at the outer diameter end.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a foil bearing, and more particularly to a thrust foil bearing in which a bearing surface is formed by arranging a plurality of leaves in the circumferential direction. [Background technology]

[0002] Bearings that support the main shafts of turbomachinery, such as gas turbines and turbochargers, are required to withstand harsh environments, including high temperatures and high rotational speeds. Foil bearings have attracted attention as a bearing suitable for use under such harsh conditions. Foil bearings have bearing surfaces made of thin, flexible foils (leaves) with low bending stiffness. During shaft rotation, a fluid film (e.g., an air film) forms in the bearing gap between the shaft and the leaf bearing surface, providing non-contact support for the shaft. Foil bearings have the advantage of being able to stably support the shaft even under harsh conditions because the leaf-based bearing surface allows for deflection and allows the bearing surface to deform in response to shaft displacement, thermal expansion, and other factors.

[0003] These foil bearings are broadly divided into radial foil bearings, which support radial loads, and thrust foil bearings, which support thrust loads. The configuration of a thrust foil bearing is shown in Figures 11 and 12. Note that Figure 11 is a plan view of the bearing surface as seen from the axial direction, and Figure 12 is an enlarged plan view of one leaf.

[0004] As shown in Figures 11 and 12, the thrust foil bearing has leaves 100 arranged at multiple locations in the rotational direction R, and a top foil portion 102 having a bearing surface S1 is formed in the area including the front end 101 of each leaf, and a back foil portion 104 is formed in the area including the rear end 103 and is arranged behind the top foil portion 102 of the adjacent leaf.

[0005] In conventional thrust foil bearings, the circumferential pitch of the leaves 100 near the inner diameter end is smaller than the circumferential pitch near the outer diameter end. As a result, the rigidity near the inner diameter end is higher than the rigidity near the outer diameter end. As a result, the area near the inner diameter end is less flexible and more likely to come into contact with other components (for example, the thrust collar 106 (see FIG. 13)).

[0006] Therefore, in the past, the angle (central angle) occupied by the inner diameter ends of the overlapping portions of adjacent leaves 100 was set smaller than the angle (central angle) occupied by the outer diameter ends (see Patent Document 1). In this case, the rigidity near the inner diameter ends can be reduced, allowing the leaves 100 to bend approximately uniformly across their entire surfaces. This allows the floating clearance h1 (see FIG. 13) to be set smaller, thereby increasing the load capacity. Here, the floating clearance h refers to the minimum width of the bearing clearance Ca (see FIG. 13), and the load capacity of the thrust foil bearing depends on this floating clearance h1. In other words, the smaller the floating clearance h, the greater the load capacity of the thrust foil bearing. Therefore, to increase the load capacity of the thrust foil bearing, it is necessary to minimize the floating clearance h1. Note that the bearing clearance Ca refers to the gap between the bearing surface S1 and the thrust collar 106, as shown in FIG. 13. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2020-159450 Summary of the Invention [Problem to be solved by the invention]

[0008] In the conventional thrust foil bearing of Patent Document 1, the angular difference between the central angle of arc portion 107 at the outer diameter end and the central angle of arc portion 108 at the inner diameter end is 10° or more. In other words, when the central angle of arc portion 107 at the outer diameter end is α and the central angle of arc portion 108 at the inner diameter end is β, α-β≧10°. However, if the difference is 10° or more, the rigidity near the inner diameter end is reduced too much, which could actually result in a decrease in load capacity.

[0009] Furthermore, in the conventional thrust foil bearing of Patent Document 1, a floating gap is formed between the tip of the top foil and the thrust collar. However, if the rigidity near the inner diameter end is reduced too much, the gap between the inner diameter end of the leaf and the thrust collar becomes large, reducing the load capacity.

[0010] In view of the above problems, the present invention provides a thrust foil bearing that can increase the load capacity by forming floating gaps in three locations: near the tip of the top foil portion, near the arc portion at the inner diameter end, and near the arc portion at the outer diameter end. [Means for solving the problem]

[0011] The foil bearing of the present invention is a foil bearing in which a bearing surface is formed by arranging a plurality of leaves in a circumferential direction, the leaves having a top foil portion that forms the bearing surface, the inner diameter end and outer diameter end of the leaf being arc portions, and by specifying the ratio between the diameter dimension of the arc portion at the inner diameter end of the leaf and the diameter dimension of the arc portion at the outer diameter end of the leaf, and the ratio between the central angle of the arc portion at the inner diameter end and the central angle of the arc portion at the outer diameter end, floating gaps are formed at three locations: near the tip of the top foil portion, near the arc portion at the inner diameter end, and near the arc portion at the outer diameter end.

[0012] According to the foil bearing of the present invention, the load capacity can be increased by forming floating gaps in three locations: near the tip of the top foil portion, near the arc portion of the inner diameter end, and near the arc portion of the outer diameter end.

[0013] It is preferable that the ratio of the diameter of the arc portion at the inner diameter end of the leaf to the diameter of the arc portion at the outer diameter end of the leaf is 1.9 to 2.1, and the ratio of the central angle of the arc portion at the inner diameter end to the central angle of the arc portion at the outer diameter end is greater than 1.0 and not more than 1.1. By setting them in this way, floating gaps can be stably formed at three locations: near the tip of the top foil portion, near the arc portion at the inner diameter end, and near the arc portion at the outer diameter end.

[0014] It is preferable that during operation, a bearing gap is formed between the bearing surface and the thrust collar, with the central portion of the top foil portion being concave toward the thrust collar, and the arc-shaped portions of the inner diameter end and the outer diameter end being convex toward the thrust collar. By setting it in this manner, floating gaps can be stably formed in three locations: near the tip of the top foil portion, near the arc-shaped portion of the inner diameter end, and near the arc-shaped portion of the outer diameter end.

[0015] The leaf may have a portion that slides against the thrust collar, and a sliding mark may be formed at the portion. By setting in this manner, the rigidity of the leaf at a portion where the rigidity is high can be intentionally reduced.

[0016] The sliding marks are preferably larger near the arc portion of the outer diameter end than near the arc portion of the inner diameter end, thereby enabling the rigidity of the high-rigidity portion to be stably reduced. [Effects of the Invention]

[0017] In the present invention, floating gaps can be formed in three locations: near the tip of the top foil portion, near the arc portion at the inner diameter end, and near the arc portion at the outer diameter end, thereby increasing the load capacity. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram of a gas turbine. [Figure 2] FIG. 2 is a cross-sectional view showing a support structure for a main shaft of a gas turbine. [Figure 3]FIG. 2 is a cross-sectional view of a thrust foil bearing incorporated into a support structure for a main shaft of a gas turbine. [Figure 4] FIG. 2 is a front view of a thrust foil bearing. [Figure 5] FIG. 2 is an enlarged front view of a leaf of a thrust foil bearing. [Figure 6] FIG. 5 is an enlarged cross-sectional view taken along line XX in FIG. 4. [Figure 7] FIG. 2 is a perspective view showing the shape of a leaf during operation. [Figure 8] 1 shows the relationship between the leaf and the thrust collar, where (a) is a simplified cross-sectional view when the gap between the inner diameter portion and the thrust collar is narrow, (b) is a simplified cross-sectional view when the gap between the inner diameter portion and the thrust collar and the gap between the outer diameter portion and the thrust collar are the same, and (c) is a simplified cross-sectional view when the gap between the outer diameter portion and the thrust collar is narrow. [Figure 9] The figures show the main parts of thrust foil bearings after break-in, where (a) is an enlarged external view of the main parts of Example 1, (b) is an enlarged external view of the main parts of Example 2, (c) is an enlarged external view of the main parts of Example 3, (d) is an enlarged external view of the main parts of Example 4, and (e) is an enlarged external view of the main parts of Example 5. [Figure 10] FIG. 1 is a simplified diagram of an operating machine used for break-in operation. [Figure 11] FIG. 1 is a plan view of a main portion of a conventional thrust foil bearing. [Figure 12] FIG. 1 is an enlarged plan view of a leaf of a conventional thrust foil bearing. [Figure 13] FIG. 12 is an enlarged cross-sectional view taken along line XX in FIG. 11. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

[0020] Figure 1 conceptually shows 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. The turbine 1, compressor 2, and generator 3 are provided with a common main shaft 6 that extends horizontally, and this main shaft 6, the turbine 1, and the compressor 2 form a rotor that can rotate integrally.

[0021] Air drawn in through intake port 7 is compressed by compressor 2, heated in regenerator 5, and then sent to combustor 4. Fuel is mixed with this compressed air and burned, and the high-temperature, high-pressure gas rotates turbine 1. The rotational force of turbine 1 is transmitted to generator 3 via main shaft 6, which rotates generator 3 to generate electricity, which is then output via inverter 8. Because the gas after rotating turbine 1 is relatively hot, this gas is sent to regenerator 5 where it exchanges heat with the compressed air before combustion, thereby reusing the heat of the gas after combustion. After heat exchange in regenerator 5, the gas passes through exhaust heat recovery device 9 before being emitted as exhaust gas.

[0022] An example of a rotor support structure for the gas turbine is shown in Figure 2. In this support structure, radial bearings 10 are arranged at two locations in the axial direction, and thrust bearings 20, 20 are arranged on both axial sides of a thrust collar 6a provided on the main shaft 6. The radial bearings 10 and thrust bearings 20 support the main shaft 6 so that it can rotate freely in the radial direction and both thrust directions.

[0023] In this support structure, the area between the turbine 1 and compressor 2 is adjacent to the turbine 1, which is rotated by high-temperature, high-pressure gas, and therefore is a high-temperature atmosphere. In this high-temperature atmosphere, lubricants such as oil and grease deteriorate and evaporate, making it difficult to apply ordinary bearings (rolling bearings, etc.) that use such lubricants. For this reason, air dynamic bearings, and in particular the foil bearing (thrust foil bearing 20) of the present invention, are suitable as bearings 10 and 20 used in this type of support structure.

[0024] Next, the configuration of a foil bearing (thrust foil bearing) suitable for the thrust bearing for the gas turbine will be described.

[0025] As shown in Figure 3, the thrust foil bearing 20 has a disk-shaped foil holder 21 and multiple leaves 22 attached to an end surface 21a of the foil holder 21. In this embodiment, the thrust foil bearings 20 are provided on both axial sides of the thrust collar 6a. These thrust foil bearings 20 have an axially symmetrical structure with the thrust collar 6a at the center. Note that hereinafter, the downstream side of the leaf 22 in the direction of fluid flow when the main shaft 6 is rotating will be referred to as the "downstream side," and the opposite side will be referred to as the "upstream side."

[0026] Foil holder 21 is made of metal, resin, or the like. Foil holder 21 is in the shape of a hollow disk having an inner hole 21b into which main shaft 6 is inserted. A plurality of leaves 22 are attached to one end surface 21a of foil holder 21. The other end surface 21c of foil holder 21 is fixed to the housing of equipment (a gas turbine in this embodiment) into which thrust foil bearing 20 is incorporated.

[0027] The leaves 22 are formed from a metal that is highly springy and easy to process, such as steel or a copper alloy. The leaves 22 are formed from a thin metal plate (foil) with a thickness of approximately 20 μm to 200 μm. In an air dynamic bearing that uses air as a fluid film as in this embodiment, since there is no oil in the atmosphere, it is preferable to form the leaves 22 from stainless steel or bronze.

[0028] The leaves 22 are arranged side by side in the circumferential direction with their phases shifted, as shown in Fig. 4. As shown in Fig. 5, each leaf 22 has a main body portion 22c made up of a top foil portion 22a having a bearing surface S and a back foil portion 22b provided continuously upstream of the top foil portion 22a.

[0029] As shown in Fig. 4, the leaves 22 are arranged at equal pitches at multiple locations in the rotation direction R (circumferential direction). Fig. 5 shows only one leaf 22 of the multiple leaves 22 arranged in the rotation direction R, with the other leaves not shown.

[0030] Each leaf 22 has a front end 221 located at the end on the rotational direction R side and a rear end 222 located at the end on the counter-rotational direction side. The front end 221 and the rear end 222 are formed in a so-called herringbone shape, with the region between both ends of the front end 221 formed in a convex shape protruding in the rotational direction R, and the rear end 222 formed in a concave shape recessed in the rotational direction R between both ends. The front end 221 and the rear end 222 have apexes 221a, 222a in approximately their radial central regions. By forming the front end 221 and the rear end 222 in this herringbone shape, it is possible to obtain the effect of drawing fluid (e.g., air) into the radial central region of the thrust bearing gap during rotation of the main shaft 6, thereby increasing the load capacity of the thrust foil bearing.

[0031] An arc portion 22d is provided at the outer diameter end of the main body portion 22c (consisting of the top fall 22a and the back fall 22b) of the leaf 22, and an arc portion 22e is provided at the inner diameter end of the main body portion 22c. Both of the arc portions 22d and 22e are centered on the rotation center O of the main shaft 6.

[0032] In this case, the ratio of the diameter of the arc portion 22e at the inner diameter end of the leaf 22 to the diameter of the arc portion 22d at the outer diameter end of the leaf 22 is 1.9 to 2.1, and the ratio of the central angle (the angle occupied by the arc portion 22e) at the inner diameter end to the central angle (the angle occupied by the arc portion 22d) at the outer diameter end is greater than 1.0 and not more than 1.1. That is, when the diameter (diameter φ) of the arc portion 22d at the outer diameter end of the leaf 22 is D1 and the diameter (diameter φ) of the arc portion 22e at the inner diameter end of the leaf 22 is D2, (D1÷D2) is 2±0.1, and when the central angle (deg) of the arc portion 22d at the outer diameter end of the leaf 22 is A and the central angle (deg) of the arc portion 22e at the inner diameter end of the leaf 22 is B, the relationship 1<(A÷B)≦1.1 is satisfied.

[0033] 4, and in this manner, when the leaves 22 are attached to the foil holder 21, the bearing surface S provided on the top foil portion 22a of each leaf 22 directly faces the thrust collar 6a in the axial direction, and the back foil portion 22b of the adjacent leaf 22 on the downstream side is disposed behind the top foil portion 22a of each leaf 22 (on the opposite side to the bearing surface S). In other words, the back foil portion 22b of each leaf 22 is disposed between the foil holder 21 and the top foil portion 22a of the adjacent leaf 22 on the upstream side.

[0034] When the main shaft 6 rotates in one circumferential direction (the direction of arrow R in Figure 4), a bearing clearance C is formed between the bearing surface S of each leaf 22 of the thrust foil bearing 20 and the end face of the thrust collar 6a. As each leaf 22 rides up onto the adjacent leaf 22 and curves, the bearing clearance C forms a wedge shape that narrows toward the downstream side (in Figure 6, each leaf 22 is simplified as a flat plate). Air in the large gap portion C1 of this wedge-shaped bearing clearance C is forced into the small gap portion C2, increasing the pressure of the air film in the bearing clearance C. This pressure provides noncontact support for the main shaft 6 in the thrust direction. The leaves 22 elastically deform in response to operating conditions such as the load, the rotational speed of the main shaft 6, and the ambient temperature, automatically adjusting the bearing clearance C to an appropriate width depending on the operating conditions. Therefore, even under harsh conditions such as high temperatures and high rotation speeds, the bearing clearance C can be maintained at an optimal width, enabling stable support of the main shaft 6.

[0035] By setting the bearing surface S and the thrust collar 6a in this manner, a bearing clearance C is formed between the bearing surface S and the thrust collar 6a during operation, and as shown in Fig. 7, the central portion of the top foil portion is concave toward the thrust collar 6a, while the inner diameter end circular arc portion 22e and the outer diameter end circular arc portion 22d are convex toward the thrust collar 6a. Therefore, floating clearances h are formed in three locations: near the tip of the top foil portion 22a, near the inner diameter end circular arc portion 22e, and near the outer diameter end circular arc portion 22d. That is, by specifying the ratio between the radial dimension of the inner diameter end circular arc portion 22e of the leaf 22 and the radial dimension of the outer diameter end circular arc portion 22d of the leaf 22, and the ratio between the central angle of the inner diameter end circular arc portion 22e and the outer diameter end circular arc portion 22d, the floating clearances h can be formed in the three locations. In this case, the leaf 22 has a portion that slides against the thrust collar 6a, and sliding marks may be formed in that portion, and these sliding marks are larger near the arc portion 22d at the outer diameter end than near the arc portion 22e at the inner diameter end.

[0036] In this way, if floating gaps h are formed in three locations, namely near the tip of the top foil portion 22, near the arc portion 22e at the inner diameter end, and near the arc portion 22d at the outer diameter end, the load capacity can be increased stably.

[0037] It is preferable that the ratio of the diameter of the arc portion 22e at the inner diameter end of the leaf 22 to the diameter of the arc portion 22d at the outer diameter end of the leaf 22 is 1.9 to 2.1, and the ratio of the central angle of the arc portion 22e at the inner diameter end to the central angle of the arc portion 22d at the outer diameter end is greater than 1.0 and not more than 1.1. By setting in this manner, floating gaps can be stably formed at three locations: near the tip of the top foil portion 22a, near the arc portion 22e at the inner diameter end, and near the arc portion 22d at the outer diameter end.

[0038] During operation, a bearing clearance C is formed between the bearing surface S and the thrust collar 6a, and it is preferable that the central portion of the top foil portion 22a be concave toward the thrust collar 6a, and the inner diameter end circular arc portion 22e side and the outer diameter end circular arc portion 22d side be convex toward the thrust collar 6a. By setting it in this way, floating clearances h can be stably formed in three locations: near the tip of the top foil portion 22a, near the inner diameter end circular arc portion 22e, and near the outer diameter end circular arc portion 22d.

[0039] The leaf 22 may have a portion that slides against the thrust collar 6a, and a sliding mark may be formed at that portion. By setting it in this way, the rigidity of the leaf at a portion where the rigidity is high can be intentionally reduced.

[0040] The sliding marks are preferably larger near the arc portion 22d of the outer diameter end than near the arc portion 22e of the inner diameter end, thereby making it possible to stably reduce the rigidity of the high-rigidity portion.

[0041] While an embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and various modifications are possible, and the application of the thrust foil bearing described above is not limited to the gas turbine described above, but can also be used, for example, as a bearing supporting the rotor of a turbocharger. Furthermore, the thrust foil bearing described above is not limited to turbomachinery such as gas turbines and turbochargers, and can be widely used as bearings for automobiles and other vehicles used under constraints such as where lubrication with liquids such as lubricating oil is difficult, where it is difficult from the standpoint of energy efficiency to provide separate accessories for a lubricating oil circulation system, or where resistance due to shearing of the liquid is a problem, and further as bearings for industrial equipment.

[0042] Furthermore, although the thrust foil bearings described above are air dynamic pressure bearings that use air as the pressure-generating fluid, this is not limiting and other gases, or liquids such as water or oil, can also be used as the pressure-generating fluid. Furthermore, while the description has been given of a case in which the shaft member is rotated, the thrust foil bearings described above can also be used in the opposite case in which the leaf side is rotated. [Example]

[0043] Next, thrust foil bearings with an outer diameter of 77 mm, an inner diameter of 39 mm, and eight leaves were created, examples 1 to 5 in Table 1, and their load capacities at 25,000 rpm were investigated. [Table 1] Examples 1 to 5 have an outer diameter of 77 mm, an inner diameter of 39 mm, and an inner / outer diameter ratio of 1.97. Example 1 has an outer diameter angle (central angle A of the arc portion at the outer diameter end) of 43.6° and an inner diameter angle (central angle B of the arc portion at the inner diameter end) of 43.7°, with an angle ratio of 1.00. Example 2 has an outer diameter angle (central angle A of the arc portion at the outer diameter end) of 43.6° and an inner diameter angle (central angle B of the arc portion at the inner diameter end) of 42.8°, with an angle ratio of 1.02. Example 3 has an outer diameter angle (central angle A of the arc portion at the outer diameter end) of 43.6° and an inner diameter angle (central angle B of the arc portion at the inner diameter end) of 41.8°, with an angle ratio of 1.04. In Example 4, the angle of the outer diameter portion (central angle A of the arc portion at the outer diameter end) is 43.6 degrees, and the angle of the inner diameter portion (central angle B of the arc portion at the inner diameter end) is 40.9 degrees, with an angle ratio of 1.07.In Example 5, the angle of the outer diameter portion (central angle A of the arc portion at the outer diameter end) is 43.6 degrees, and the angle of the inner diameter portion (central angle B of the arc portion at the inner diameter end) is 40.0 degrees, with an angle ratio of 1.09.

[0044] The load on Model 1 was 200N, the load on Model 2 was 330N, the load on Model 3 was 350N, the load on Model 4 was 350N, and the load on Model 5 was 340N. These loads on Models 1 to 5 are those before break-in. If break-in is performed, all of Models 1 to 5 can secure the same load capacity (360N).

[0045] The break-in is performed using a break-in device such as that shown in Figure 10, which intentionally slides the highly rigid portion of the leaf 22 against the thrust collar 6a, thereby grinding down the highly rigid portion and reducing its rigidity.

[0046] The break-in device shown in FIG. 10 includes an air cylinder mechanism 31, a linear guide mechanism 32, an electric spindle motor 33, and the like, which are arranged on a base 34. The air cylinder mechanism 31 includes a cylinder body 31a and a rod 31b protruding from the cylinder body 31a. A load sensor (load cell 35) is attached to the tip of the rod 31b. A foil holder housing 36 is supported on the tip of the rod 32b of the linear guide mechanism 32. A foil bearing 99 is fixed to the end face of the foil holder housing 36 on the electric spindle motor 33 side. A test shaft 37 having a disk portion 37a is provided on the shaft end (shaft end on the foil holder housing 36 side) of the electric spindle motor 33. The disk portion 37a serves as a thrust collar.

[0047] By driving the air cylinder mechanism 31, an arbitrary load is applied to the test bearing (thrust foil bearing 99) attached to the foil holder housing 36 via the load cell 35 and linear guide mechanism 32. In this case, the test shaft 37 is driven to rotate by driving the electric spindle motor 33.

[0048] The torque of this test bearing 99 is correlated with the motor current value of the electric spindle motor 33, and the current value output from the motor driver is measured, and the load when the current value increases is the load capacity of the bearing.

[0049] Incidentally, Figure 9 shows the appearance of the leaf after the break-in test, with Figure 9(a) showing Example 1, Figure 9(b) showing Example 2, Figure 9(c) showing Example 3, Figure 9(d) showing Example 4, and Figure 9(e) showing Example 5.

[0050] As shown in Figure 9(a), in Example 1, the disk part of the test shaft (i.e., the thrust collar) slid against the inner diameter end of the leaf, causing a strong impact. As shown in Figure 9(b), in Example 2, impact occurred along the entire leaf contour. As shown in Figure 9(c), in Example 3, impact occurred on the outer diameter end of the leaf. As shown in Figure 9(d), in Example 4, impact occurred on the outer diameter end of the leaf. As shown in Figure 9(e), in Example 5, impact occurred on the outer diameter end of the leaf.

[0051] Figures 8(a), 8(b), and 8(c) are conceptual diagrams of bearings in operation, and are simplified enlarged cross-sectional views of line YY in Figure 4. Figure 8(a) shows Model 1, in which the gap between the inner diameter end and the thrust collar (the disk part of the test shaft) is narrow. This is because the peripheral speed at the inner diameter end is slow, resulting in low pressure. Figure 8(b) shows Models 2 and 3, in which the gap between the inner diameter end and the thrust collar and the gap between the outer diameter end and the thrust collar are equivalent. Figure 8(c) shows Models 4 and 5, in which the gap between the outer diameter end and the thrust collar is narrow.

[0052] The load capacity value is extremely low for Example 1, which has an angle ratio of 1.00, while the load capacities of Examples 2 to 4, which have angle ratios greater than 1.00, are high, with Examples 3 and 4 being particularly high. Also, Example 5, which has an angle ratio of 1.09, has a reduced load capacity. The low load capacity of Example 1 is due to the high rigidity of the inner diameter end, which resulted in the formation of a floating gap between the inner diameter end and the thrust collar. The low load capacity of Example 5 is due to the high rigidity of the outer diameter end, which resulted in the formation of a floating gap between the outer diameter end and the thrust collar. Comparing Example 1 and Example 5, the peripheral speed of the thrust collar is faster at the outer diameter end than at the inner diameter end, resulting in the higher load capacity of Example 5.

[0053] By performing a break-in operation, the load capacities of all of Examples 1 to 5 can be made equal. However, considering productivity, Examples 3 to 4 are preferable designs because they can obtain high load capacities without performing a break-in operation.

[0054] Furthermore, when comparing the break-in times of Example 1 and Example 5, the peripheral speed of the thrust collar is faster on the outer diameter end side than on the inner diameter end side, so Example 5 has a larger workload than Example 1, and the break-in time can be shortened. Therefore, if the angle ratio varies due to manufacturing, it is preferable to widen the tolerance on Example 5 side compared to Example 1 side. [Explanation of symbols]

[0055] C Bearing clearance S bearing surface h Floating gap 6a Thrust Collar 22 Leaf 22a Top foil section 22b Back foil section 22d, 22e Arc section

Claims

1. A foil bearing in which a bearing surface is formed by arranging a plurality of leaves in a circumferential direction, the leaf has a top foil portion that forms a bearing surface, the inner diameter end and the outer diameter end of the leaf are arc-shaped, and a plurality of leaves are arranged circumferentially with a phase difference so that they partially overlap without providing a recessed groove on the bearing surface side of the leaf, and the ratio of the diameter of the arc portion of the inner diameter end of the leaf to the diameter of the arc portion of the outer diameter end of the leaf is set to 1.9 to 2.1, and the ratio of the central angle of the arc portion of the inner diameter end to the central angle of the arc portion of the outer diameter end is set to be greater than 1.0 and not more than 1.1, thereby forming floating gaps at three locations: near the tip of the top foil portion, near the arc portion of the inner diameter end, and near the arc portion of the outer diameter end.

2. 2. The foil bearing according to claim 1, wherein during operation, a bearing gap is formed between the bearing surface and the thrust collar, the central portion of the top foil portion is concave toward the thrust collar, and the arc portion side of the inner diameter end and the arc portion side of the outer diameter end are convex toward the thrust collar.

3. 3. The foil bearing according to claim 2, wherein the leaf has a portion that slides against the thrust collar, and a sliding mark is formed on the portion.

4. The foil bearing according to claim 3, wherein the sliding marks are larger near the arc portion of the outer diameter end than near the arc portion of the inner diameter end.

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