Gas dynamic pressure thrust bearing

Abstract

The application discloses a gas dynamic pressure thrust bearing, which comprises a bearing bottom plate, a wave foil and a flat foil arranged in layers; the bearing bottom plate is in the shape of a ring plate; the wave foil and the flat foil are arranged in a ring column around the center hole of the bearing bottom plate; the outer ring boundary of the flat foil exceeds the outer ring boundary of the wave foil outward; and the inner ring boundary of the flat foil exceeds the inner ring boundary of the wave foil inward. The wave foil directly supports the flat foil; the inner and outer ring side edges of the flat foil exceed the inner and outer ring side edges of the wave foil outward, so that the inner and outer ring side edges of the flat foil are suspended relative to the wave foil to form a cantilever structure, which is favorable for improving the deformation characteristics of the inner and outer ring side edges of the flat foil, enabling the inner and outer ring side edges of the flat foil to generate sufficient deformation under specific working conditions, thereby reducing the wear degree of the inner and outer ring side edges of the flat foil, improving the gas intake between the flat foil and the wave foil, and improving the convective heat dissipation capacity of the gas dynamic pressure thrust bearing.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of bearings, in particular to a gas dynamic pressure thrust bearing. BACKGROUND

[0002] Compared with traditional rolling bearings and sliding bearings, a gas dynamic pressure bearing can use ambient gas as a lubricating medium, generate a high-pressure gas film on the bearing surface through a dynamic pressure effect, and achieve complete suspension of a rotating shaft. Therefore, the gas dynamic pressure bearing has a series of advantages such as high speed, good precision, high reliability, oil-free lubrication, small friction loss, and the ability to work in a high-temperature environment.

[0003] A gas dynamic pressure thrust bearing is a common form of a gas dynamic pressure bearing.

[0004] A widely used gas dynamic pressure thrust bearing is a wave foil type thrust bearing. The wave foil type thrust bearing is disc-shaped, and a rotating shaft is installed in a disc center hole of the wave foil type thrust bearing. Due to the influence of the external environment, the gas film pressure of the wave foil type thrust bearing presents the phenomenon that the pressure at the inner and outer edges of the disc is significantly smaller than the pressure at other positions. This limits the deformation degree of the inner and outer edges of the disc of the wave foil type thrust bearing, causes the inner and outer edges of the disc of the wave foil type thrust bearing to easily collide and rub with adjacent components, and causes external air to enter from the inner and outer edges of the disc of the wave foil type thrust bearing, thereby causing a serious local temperature rise of the wave foil type thrust bearing, and further possibly causing the failure of the wire coating of the foil and the shaft seizure phenomenon. SUMMARY

[0005] The purpose of the present application is to provide a gas dynamic pressure thrust bearing that can reduce the friction and wear of the foil, increase the air intake at the edge of the foil, and avoid a serious local temperature rise.

[0006] To achieve the above-mentioned purpose, the present application provides a gas dynamic pressure thrust bearing, which comprises a bearing bottom plate, a wave foil, and a flat foil arranged in layers; the bearing bottom plate is annular and plate-shaped, the wave foil and the flat foil are arranged in a ring around the center hole of the bearing bottom plate, the outer ring boundary of the flat foil extends outward beyond the outer ring boundary of the wave foil, and the inner ring boundary of the flat foil extends inward beyond the inner ring boundary of the wave foil.

[0007] In some embodiments, the outer ring edge of the bearing bottom plate extends outward beyond the outer ring boundary of the flat foil, and the inner ring edge of the bearing bottom plate extends inward beyond the inner ring boundary of the flat foil.

[0008] In some embodiments, the wave foil comprises a plurality of fan-shaped ring wave foils, and all the fan-shaped ring wave foils are arranged in a ring around the center hole of the bearing bottom plate.

[0009] The flat foil comprises a plurality of fan-shaped ring flat foils, and all the fan-shaped ring flat foils are arranged in a ring around the center hole of the bearing bottom plate.

[0010] In some embodiments, the axial projection of one sector-shaped wave foil is within the axial projection of one sector-shaped flat foil; the axial projection of all sector-shaped wave foils corresponds to the axial projection of all sector-shaped flat foils one by one.

[0011] In some embodiments, the two sector-shaped side edges of any sector-shaped flat foil are respectively a first flat foil side edge fixedly connected to the bearing bottom plate and a second flat foil side edge overlapped with the bearing bottom plate.

[0012] In some embodiments, the two sector-shaped side edges of any sector-shaped wave foil are respectively a first wave foil side edge fixedly connected to the bearing bottom plate and a second wave foil side edge overlapped with the bearing bottom plate; in a pair of sector-shaped wave foils and sector-shaped flat foils whose axial projections overlap with each other, the first wave foil side edge and the first flat foil side edge are located at different sides, and the second wave foil side edge and the second flat foil side edge are located at different sides.

[0013] In some embodiments, the bearing bottom plate is provided with a plurality of bottom plate grooves, any bottom plate groove is distributed along the radial direction of the bearing bottom plate, and all bottom plate grooves are uniformly arranged in a ring shape along the circumferential direction of the bearing bottom plate; the bearing bottom plate is provided with a plurality of bottom plate spacers, all bottom plate spacers are in close proximity to all bottom plate grooves one by one; the number of all bottom plate grooves, the number of all sector-shaped wave foils and the number of all sector-shaped flat foils are equal, and any sector-shaped wave foil and any sector-shaped flat foil are located between two adjacent bottom plate grooves.

[0014] In some embodiments, the distance between the bearing bottom plate and any sector-shaped flat foil gradually increases from the first wave foil side edge to the second wave foil side edge; in the same flat foil, the direction extending from the first wave foil side edge to the second wave foil side edge is the rotation direction of the gas dynamic pressure thrust bearing.

[0015] In some embodiments, any sector-shaped wave foil is provided with a plurality of foil grooves, any foil groove is arc-shaped and extends along the circumferential direction of the sector-shaped wave foil, and all foil grooves are distributed in the radial direction of the sector-shaped wave foil; any foil groove is open to the sector-shaped side edge of the sector-shaped wave foil.

[0016] The same sector-shaped wave foil is divided into multiple segments by all foil grooves, and the length of each segment of the same sector-shaped wave foil gradually decreases from the axial middle part of the bearing sleeve to the axial two ends.

[0017] With respect to the above background technology, the gas dynamic pressure thrust bearing provided by the present application comprises a bearing bottom plate, a wave foil and a flat foil which are stacked layer by layer; wherein the bearing bottom plate is in the form of a ring-shaped plate, the wave foil and the flat foil are arranged in a ring shape around the center hole of the bearing bottom plate; the outer ring boundary of the flat foil exceeds the outer ring boundary of the wave foil outward, and the inner ring boundary of the flat foil exceeds the inner ring boundary of the wave foil inward.

[0018] In the gas dynamic pressure thrust bearing, the bearing bottom plate directly supports the wave foil, and the wave foil directly supports the flat foil. Since the inner and outer ring boundaries of the flat foil respectively exceed the inner and outer ring boundaries of the wave foil, it can be seen that the inner and outer ring boundaries of the flat foil are suspended relative to the wave foil, so that the inner and outer ring boundaries of the flat foil form a cantilever structure, and therefore, the inner and outer ring boundaries of the flat foil are more prone to deformation. When the gas dynamic pressure thrust bearing starts, stops or runs under variable load, the inner and outer ring boundaries of the flat foil can easily produce sufficient deformation, avoiding excessive wear of the inner and outer ring boundaries of the flat foil, and also improving the gas inlet amount between the flat foil and the wave foil to improve the convective heat dissipation effect. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor on the basis of the provided drawings.

[0020] Figure 1 The structural schematic diagram of the gas dynamic pressure thrust bearing provided by the embodiments of the present application;

[0021] Figure 2 The exploded view of the gas dynamic pressure thrust bearing provided by the embodiments of the present application;

[0022] Figure 3 The structural schematic diagram of the bearing bottom plate provided by the embodiments of the present application;

[0023] Figure 4 The assembly schematic diagram of the fan ring wave foil and the fan ring flat foil provided by the embodiments of the present application;

[0024] Figure 5 The assembly schematic diagram of the bearing bottom plate, the fan ring wave foil, the fan ring flat foil and the bottom plate gasket provided by the embodiments of the present application.

[0025] Wherein, 1-bearings bottom plate, 11-bottom plate groove, 2-wave foil, 21-fan ring wave foil, 211-first wave foil side, 212-second wave foil side, 213-foil groove, 3-flat foil, 31-fan ring flat foil, 311-first flat foil side, 312-second flat foil side, 4-bottom plate gasket. DETAILED DESCRIPTION

[0026] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all the embodiments of the present application. Based on the embodiments of the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative work are within the scope of protection of the present application.

[0027] In order for those skilled in the art to better understand the technical solutions of the present application, the present application will be further described in detail below with reference to the drawings and specific embodiments.

[0028] Please refer to Figures 1 to 5 , Figure 1 The structural schematic diagram of the gas dynamic pressure thrust bearing provided by the embodiments of the present application is shown in the figure. Figure 2 The exploded view of the gas dynamic pressure thrust bearing provided by the embodiments of the present application is shown in the figure. Figure 3 The structural schematic diagram of the bearing bottom plate provided by the embodiments of the present application is shown in the figure.

[0029] Figure 4 The assembly schematic diagram of the fan ring wave foil and the fan ring flat foil provided by the embodiments of the present application is shown in the figure. Figure 5 The assembly schematic diagram of the bearing bottom plate, the fan ring wave foil, the fan ring flat foil and the bottom plate gasket provided by the embodiments of the present application is shown in the figure.

[0030] Please refer to Figure 1 and Figure 2 The present application provides a gas dynamic pressure thrust bearing, which comprises a bearing bottom plate 1, a wave foil 2 and a flat foil 3. The bearing bottom plate 1, the wave foil 2 and the flat foil 3 are stacked one by one to form a ring structure for a rotating shaft to pass through. In the gas dynamic pressure thrust bearing, the bearing bottom plate 1 is in the form of a ring plate, and the wave foil 2 and the flat foil 3 are arranged in a ring around the center hole of the bearing bottom plate 1. The outer ring boundary of the aforementioned flat foil 3 is outwardly beyond the outer ring boundary of the wave foil 2, and the inner ring boundary of the aforementioned flat foil 3 is inwardly beyond the inner ring boundary of the wave foil 2.

[0031] When the gas dynamic pressure thrust bearing and the rotating shaft are assembled, the rotating shaft passes through the center hole of the bearing bottom plate 1. Therefore, in this embodiment, the wave foil 2 and the flat foil 3 are arranged in a ring around the center hole of the bearing bottom plate 1, which meets the installation requirements of the rotating shaft.

[0032] The corrugated foil 2 is positioned between the bearing base plate 1 and the flat foil 3; in other words, the flat foil 3 is supported by the corrugated foil 2. The outer ring boundary of the flat foil 3 extends outward beyond the outer ring boundary of the corrugated foil 2, thus suspending the outer ring boundary of the flat foil 3 relative to the corrugated foil 2. Similarly, the inner ring boundary of the flat foil 3 extends inward beyond the inner ring boundary of the corrugated foil 2, also suspending the inner ring boundary of the flat foil 3 relative to the corrugated foil 2. Therefore, both the inner and outer edges of the flat foil 3 form cantilever structures, which facilitates deformation of the inner and outer edges of the flat foil 3 under specific operating conditions, thereby reducing wear on the flat foil 3. These specific operating conditions generally refer to the starting, stopping, and variable load operation of the gas dynamic thrust bearing.

[0033] The gas dynamic thrust bearing provided in this application will be further described below with reference to the accompanying drawings and embodiments.

[0034] In some embodiments, reference may be made to Figure 1 The axial projections of both the corrugated foil 2 and the flat foil 3 lie within the axial projection of the bearing base plate 1. In other words, the outer ring edge of the bearing base plate 1 extends outward beyond the outer ring boundary of the flat foil 3, and the inner ring edge of the bearing base plate 1 extends inward beyond the inner ring boundary of the flat foil 3. As previously known, both the inner and outer edges of the flat foil 3 are suspended above the corrugated foil 2. Therefore, the outer ring edge of the bearing base plate 1 extends outward beyond the outer ring boundary of the corrugated foil 2, and the inner ring edge of the bearing base plate 1 extends inward beyond the inner ring boundary of the corrugated foil 2. It is evident that the inner diameter of the bearing base plate 1 is smaller than the inner diameter of both the corrugated foil 2 and the flat foil 3; the outer diameter of the bearing base plate 1 is larger than both the outer diameter of the corrugated foil 2 and the outer diameter of the flat foil 3. The corrugated foil 2 directly supports the flat foil 3, while the bearing base plate 1 indirectly supports the flat foil 3 through the corrugated foil 2. It can also limit the deformation range of the inner and outer edges of the flat foil 3, so as to avoid excessive deformation of the flat foil 3 and affect its normal operating characteristics.

[0035] For reference Figures 1 to 4 The aforementioned corrugated foil 2 may include multiple fan-shaped annular corrugated foils 2, all of which are arranged in a ring around the central hole of the bearing base plate 1. Typically, the inner ring edges of all the fan-shaped annular corrugated foils 2 are on the same circumference, and the outer ring edges of all the fan-shaped annular corrugated foils 2 are on the same circumference. Similar to the corrugated foil 2, the flat foil 3 may include multiple fan-shaped flat foils 31, all of which are arranged in a ring around the central hole of the bearing base plate 1. Typically, the inner ring edges of all the fan-shaped flat foils 31 are on the same circumference, and the outer ring edges of all the fan-shaped flat foils 31 are on the same circumference.

[0036] The single fan-shaped flat foil 31 of the flat foil 3 is fan-shaped and small in size, which is conducive to the deformation of its inner and outer ring edges under specific working conditions. This deformation helps to reduce wear and increase the amount of gas intake to improve the convective heat dissipation effect.

[0037] On the basis of the above-mentioned embodiments, all the sector-annular wave foils 21 and all the sector-annular flat foils 31 are installed in pairs, and obviously, the number of the sector-annular wave foils 21 is the same as that of the sector-annular flat foils 31. When one pair of the sector-annular wave foil 21 and the sector-annular flat foil 31 are installed on the bearing bottom plate 1, the axial projection of the sector-annular wave foil 21 is within the axial projection of the sector-annular flat foil 31.

[0038] According to the shape and size relationship between the sector-annular wave foil 21 and the sector-annular flat foil 31, the axial projection of the sector-annular wave foil 21 within the axial projection of the sector-annular flat foil 31 can include the following cases:

[0039] (1) The central angle of the sector-annular wave foil 21 is equal to that of the sector-annular flat foil 31. As can be seen, when one pair of the sector-annular wave foil 21 and the sector-annular flat foil 31 are installed on the bearing bottom plate 1, the two sector-annular sides of the sector-annular wave foil 21 are respectively aligned with the two sector-annular sides of the sector-annular flat foil 31. Of course, according to the foregoing, the inner and outer sides of the sector-annular flat foil 31 respectively exceed the inner and outer sides of the sector-annular wave foil 21.

[0040] (2) The central angle of the sector-annular wave foil 21 is smaller than that of the sector-annular flat foil 31. When one pair of the sector-annular wave foil 21 and the sector-annular flat foil 31 are installed on the bearing bottom plate 1, the two sector-annular sides of the sector-annular flat foil 31 can both exceed the two sector-annular sides of the sector-annular wave foil 21, in addition, the sector-annular flat foil 31 can only have one sector-annular side that can exceed the sector-annular side of the sector-annular wave foil 21.

[0041] When one pair of the sector-annular wave foil 21 and the sector-annular flat foil 31 are installed on the bearing bottom plate 1, the two sector-annular sides of the sector-annular flat foil 31 are respectively a first flat foil side 311 and a second flat foil side 312, the aforementioned first flat foil side 311 is fixedly connected to the bearing bottom plate 1, and the aforementioned second flat foil side 312 is overlapped on the bearing bottom plate 1.

[0042] When the fan-shaped flat foil 31 is connected to the bearing bottom plate 1 in the above-mentioned mounting manner, the fan-shaped wave foil 21 can be clamped between the fan-shaped wave foil 21 and the bearing bottom plate 1. In addition, the fan-shaped wave foil 21 can also be connected to the bearing bottom plate 1 in a similar mounting manner as the fan-shaped flat foil 31, for example, the two fan-shaped sides of any one fan-shaped wave foil 21 are a first wave foil side 211 and a second wave foil side 212, the first wave foil side 211 is fixedly connected to the bearing bottom plate 1, and the second wave foil side 212 overlaps the bearing bottom plate 1. Among the pair of fan-shaped wave foils 21 and the fan-shaped flat foils 31, the first wave foil side 211 and the first flat foil side 311 are located on opposite sides, and the second wave foil side 212 and the second flat foil side 312 are located on opposite sides.

[0043] On the basis of the above-mentioned embodiments, the gas dynamic pressure thrust bearing provided by the present application further comprises a plurality of bottom plate spacers 4, any one of which can be in the shape of an elongated strip; the bearing bottom plate 1 is provided with a plurality of bottom plate grooves 11, any one of which extends along the radial direction of the bearing bottom plate 1, and all of the bottom plate grooves 11 are uniformly arranged in a ring along the circumferential direction of the bearing bottom plate 1; all of the bottom plate spacers 4 correspondingly and immediately abut all of the bottom plate grooves 11, and any one of the bottom plate spacers 4 is fixed to the bearing bottom plate 1. In addition, in the gas dynamic pressure thrust bearing, the number of all of the bottom plate grooves 11 is equal to the number of all of the fan-shaped wave foils 21 and the number of all of the fan-shaped flat foils 31; any one of the fan-shaped wave foils 21 is located between adjacent bottom plate grooves 11 and between adjacent bottom plate spacers 4, and any one of the fan-shaped flat foils 31 is located between adjacent bottom plate grooves 11 and between adjacent bottom plate spacers 4.

[0044] In some embodiments, the distance between any one fan-shaped flat foil 31 and the bearing bottom plate 1 gradually increases from the first wave foil side 211 to the second wave foil side 212, so that the fan-shaped flat foil 31 forms a wedge surface, as shown in Figure 5 . Among them, the transition direction of any one fan-shaped flat foil 31 along its circumferential direction from the first wave foil side 211 to the second wave foil side 212 is also the rotation direction of the gas dynamic pressure thrust bearing, therefore, the above-mentioned wedge surface is beneficial to the formation of a high-pressure gas film when the gas dynamic pressure thrust bearing is running.

[0045] For reference Figure 4In some other embodiments provided in the present application, any one of the sector-annular wave foil 21 is provided with a plurality of foil grooves 213, each of the foil grooves 213 is arc-shaped and extends along the circumferential direction of the sector-annular wave foil 21, and all the foil grooves 213 are distributed in the radial direction of the sector-annular wave foil 21. Of course, any one of the aforementioned foil grooves 213 is open to at most one of the sector-annular sides of the sector-annular wave foil 21, for example, the two sector-annular sides of the sector-annular wave foil 21 include the first wave foil side 211 and the second wave foil side 212, and all the foil grooves 213 can be open to the second wave foil side 212 of the sector-annular wave foil 21.

[0046] For the same sector-annular wave foil 21, it is divided into a plurality of segments by all the foil grooves 213, for example, one sector-annular wave foil 21 includes four foil grooves 213, and one sector-annular wave foil 21 is divided into five segments by the four foil grooves 213; wherein the length of each segment of the same sector-annular wave foil 21 gradually decreases from the axial middle part to the axial two ends, that is, the closer to the inner side and the outer side of the sector-annular wave foil 21, the smaller the distance between the adjacent two foil grooves 213, and vice versa. The aforementioned segmentation feature of the sector-annular wave foil 21 can greatly reduce the rigidity of the radial two ends of the sector-annular wave foil 21, thus being beneficial to reducing the difficulty of deformation of the inner and outer sides of the sector-annular wave foil 21, making the inner and outer sides of the sector-annular wave foil 21 more easily elastically deformed, and further ensuring that the inner and outer sides of the sector-annular wave foil 31 can produce a large enough deformation amount under a specific working condition.

[0047] The gas dynamic pressure thrust bearing provided in the present application is described in detail above. In this paper, specific examples are used to illustrate the principles and implementation methods of the present application. The above description of the embodiments is only used to help understand the method of the present application and its core idea. It should be pointed out that for ordinary skilled persons in the technical field, some improvements and modifications can be made to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application.

Claims

1. A gas dynamic pressure thrust bearing, characterized by, The bearing comprises a bearing base plate (1), a wave foil (2) and a flat foil (3) which are stacked layer by layer; the bearing base plate (1) is in the shape of a ring plate, the wave foil (2) and the flat foil (3) are arranged in a ring around the center hole of the bearing base plate (1), the outer ring boundary of the flat foil (3) exceeds the outer ring boundary of the wave foil (2), and the inner ring boundary of the flat foil (3) exceeds the inner ring boundary of the wave foil (2); The wave foil (2) comprises a plurality of sector ring wave foils (21), and all the sector ring wave foils (21) are arranged in a ring around the center hole of the bearing base plate (1); The flat foil (3) comprises a plurality of sector ring flat foils (31), and all the sector ring flat foils (31) are arranged in a ring around the center hole of the bearing base plate (1); The axial projection of one sector ring wave foil (21) is within the axial projection of one sector ring flat foil (31); the axial projections of all the sector ring wave foils (21) correspond to the axial projections of all the sector ring flat foils (31) one by one; The central angle of the sector ring wave foil (21) is equal to the central angle of the sector ring flat foil (31), or the central angle of the sector ring wave foil (21) is smaller than the central angle of the sector ring flat foil (31); The outer ring edge of the bearing base plate (1) exceeds the outer ring boundary of the flat foil (3), and the inner ring edge of the bearing base plate (1) exceeds the inner ring boundary of the flat foil (3); The two sector ring side edges of any sector ring flat foil (31) are respectively a first flat foil side edge (311) fixedly connected with the bearing base plate (1) and a second flat foil side edge (312) overlapped with the bearing base plate (1); The two sector ring side edges of any sector ring wave foil (21) are respectively a first wave foil side edge (211) fixedly connected with the bearing base plate (1) and a second wave foil side edge (212) overlapped with the bearing base plate (1); in a pair of sector ring wave foils (21) and sector ring flat foils (31) which are overlapped in axial projection, the first wave foil side edge (211) and the first flat foil side edge (311) are located at different sides, and the second wave foil side edge (212) and the second flat foil side edge (312) are located at different sides.

2. The gas dynamic pressure thrust bearing of claim 1, wherein, The bearing base plate (1) is provided with a plurality of base plate grooves (11), any base plate groove (11) is distributed along the radial direction of the bearing base plate (1), and all the base plate grooves (11) are uniformly arranged in a ring along the circumferential direction of the bearing base plate (1); the bearing base plate (1) is provided with a plurality of base plate gaskets (4), all the base plate gaskets (4) are one by one correspondingly adjacent to all the base plate grooves (11); the number of all the base plate grooves (11), the number of all the sector ring wave foils (21) and the number of all the sector ring flat foils (31) are equal, and any sector ring wave foil (21) and any sector ring flat foil (31) are located between two adjacent base plate grooves (11).

3. The gas dynamic pressure thrust bearing according to claim 1 or 2, characterized in that The distance between the bearing bottom plate (1) and any of the fan-shaped annular wave foil (31) gradually increases from the first wave foil side (211) to the second wave foil side (212); in the same wave foil (3), the direction extending from the first wave foil side (211) to the second wave foil side (212) is the rotating direction of the gas dynamic pressure thrust bearing.

4. The gas dynamic pressure thrust bearing according to claim 1 or 2, characterized in that Any of the fan-shaped annular wave foil (21) is provided with a plurality of foil grooves (213), any of the foil grooves (213) is arc-shaped and extends along the circumference of the fan-shaped annular wave foil (21), and all the foil grooves (213) are distributed in the radial direction of the fan-shaped annular wave foil (21); any of the foil grooves (213) is open to the fan-shaped annular side of the fan-shaped annular wave foil (21); The same fan-shaped annular wave foil (21) is divided into multiple sections by all the foil grooves (213), and the length of each section of the same fan-shaped annular wave foil (21) gradually decreases from the radial middle part to the radial ends of the fan-shaped annular wave foil (21).

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