Patent Publication Number: US-10781853-B2

Title: Radial foil bearing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a U.S. national stage application under 35 U.S.C. § 371 of International Patent Application No. PCT/JP2018/009943 filed on Mar. 14, 2018, and which was published in Japanese on Sep. 20, 2018, as International Publication No. WO 2018/168919 A1 under PCT Article 21(2). Priority is claimed on Japanese Patent Application No. 2017-050127, filed Mar. 15, 2017, the content of which is incorporated herein by reference. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not Applicable. 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC OR AS A TEXT FILE VIA THE OFFICE ELECTRONIC FILING SYSTEM (EFS-WEB) 
     Not Applicable. 
     STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTOR OR A JOINT INVENTOR 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     Technical Field 
     The present disclosure relates to a radial foil bearing. 
     Heretofore, as a bearing for a high speed rotating body, a radial bearing is known which is used in a state of encircling a rotary shaft. As such a radial bearing, a radial foil bearing is well known which includes a thin sheet-shaped top foil forming a bearing surface, a back foil elastically supporting the top foil, and a cylindrical bearing housing accommodating the top foil and the back foil. As the back foil of the radial foil bearing, a bump foil obtained by forming a thin sheet into a wave sheet shape is mainly used. 
     In such a radial foil bearing, an intermediate foil is inserted between the top foil and the back foil for the purpose of “improving the damping effect due to friction between foils”, “increasing the rigidity of the top foil” or the like (refer to, for example, Patent Document 1). The intermediate foil is formed into a thin sheet shape, elastically contacts the tops of hill parts of the wave sheet-shaped bump foil, and causes energy dissipation due to friction through sliding with respect to the tops, thereby damping the film-pressure fluctuation. That is, it is possible to limit axial vibration (self-excited vibration) of the rotary shaft by this damping effect and to easily settle the axial vibration. 
     DOCUMENT OF RELATED ART 
     Patent Document 
     [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2014-20463 
     BRIEF SUMMARY OF THE INVENTION 
     Technical Problem 
     An object of the present disclosure is to improve the damping effect due to the friction between the intermediate foil and the back foil described above. 
     Solution to Problem 
     In order to solve the above problems, a radial foil bearing of one aspect of the present disclosure includes: a housing provided with an insertion hole, a pair of engagement grooves extending outward in a radial direction from an inner peripheral edge of the insertion hole being provided in both end surfaces in an axial direction of the housing; a back foil disposed on an inner circumferential surface of the insertion hole; an intermediate foil supported by the back foil; and an engagement member including a pair of engagement legs engaging with the pair of engagement grooves, and a connection portion connecting the pair of engagement legs. In addition, the intermediate foil is provided with a recess recessed and protruding toward the back foil, and the connection portion is disposed in the recess. 
     In the above aspect of the present disclosure, the back foil may be formed into a wave sheet shape, and the recess may be disposed at a position in a circumferential direction between a top of at least one hill part and a top of a hill part adjacent to the one hill part in the wave sheet shape of the back foil. 
     In the above aspect of the present disclosure, a side surface of the hill part of the wave sheet shape of the back foil may be a curved surface, and a contact surface of the recess contacting the side surface may be an inclined surface. 
     In the above aspect of the present disclosure, both ends in the axial direction of each of the intermediate foil and the back foil may be provided with cutouts with which the pair of engagement legs engage. 
     Effects 
     According to the present disclosure, it is possible to improve the damping effect due to friction between an intermediate foil and a back foil. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a side view showing an example of a turbo machine to which a radial foil bearing of the present disclosure is applied. 
         FIG. 2  is a diagram showing a first embodiment of the radial foil bearing of the present disclosure. 
         FIG. 3A  is a development view of a top foil provided in the radial foil bearing shown in  FIG. 2 . 
         FIG. 3B  is a side view showing, in a flattened manner, the top foil provided in the radial foil bearing shown in  FIG. 2 . 
         FIG. 4  is a perspective view showing an important part of the radial foil bearing shown in  FIG. 2 . 
         FIG. 5A  is a diagram schematically showing, in a flattened manner, the important part of the radial foil bearing shown in  FIG. 2 . 
         FIG. 5B  is a side view showing, in a flattened manner, the important part of the radial foil bearing shown in  FIG. 2 . 
         FIG. 6  is an enlarged view of the important part shown in  FIG. 5B . 
         FIG. 7A  is a diagram showing a second embodiment of the radial foil bearing of the present disclosure and schematically showing an important part of the radial foil bearing in a flattened manner. 
         FIG. 7B  is a diagram showing the second embodiment of the radial foil bearing of the present disclosure and is a side view showing the important part of the radial foil bearing in a flattened manner. 
         FIG. 8  is an enlarged cross-sectional view taken along line A-A in  FIG. 7A . 
         FIG. 9A  is a diagram showing a third embodiment of the radial foil bearing of the present disclosure and schematically showing an important part of the radial foil bearing in a flattened manner. 
         FIG. 9B  is a diagram showing the third embodiment of the radial foil bearing of the present disclosure and is a side view showing the important part of the radial foil bearing in a flattened manner. 
         FIG. 10  is an enlarged cross-sectional view taken along line B-B in  FIG. 9A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the radial foil bearing of the present disclosure will be described in detail with reference to the drawings. 
       FIG. 1  is a side view showing an example of a turbo machine to which a radial foil bearing of the present disclosure is applied. The reference sign  1  in  FIG. 1  represents a rotary shaft, the reference sign  2  represents an impeller provided at one end in the axial direction of the rotary shaft, and the reference sign  3  represents the radial foil bearing of the present disclosure. Although only one radial foil bearing is shown in  FIG. 1  omitting another bearing, in general, two radial foil bearings are provided in the axial direction of the rotary shaft  1  to configure a support structure for the rotary shaft  1 . Therefore, two radial foil bearings  3  are also provided in this embodiment. 
     The radial foil bearing  3  encircles the rotary shaft  1 , that is, the rotary shaft  1  is inserted therethrough. A thrust collar  4  is provided on the rotary shaft  1  between the impeller  2  and the radial foil bearing  3 . Thrust bearings  5  are disposed on both sides of the thrust collar  4  in the axial direction so as to face the thrust collar  4 , that is, the shaft is inserted through the thrust bearings  5 . 
     The impeller  2  is disposed inside a housing  6  that is the stationary portion, and a tip clearance  7  is provided between the impeller  2  and the housing  6 . 
     First Embodiment 
       FIG. 2  is a diagram showing a first embodiment of the present disclosure. As shown in  FIG. 2 , a radial foil bearing  3  of the first embodiment is a cylindrical bearing provided with an insertion hole  12   a , encircling and supporting the rotary shaft  1 . A direction parallel to the central axis of the radial foil bearing  3  (namely, the central axis of the insertion hole  12   a ) is referred to as an axial direction, a direction crossing the central axis is referred to as a radial direction, and a direction around the central axis is referred to as a circumferential direction. The radial foil bearing  3  is configured including a cylindrical top foil  9  (namely, the top foil  9  encircling a circumferential side surface (outer circumferential surface) of the rotary shaft  1 ) disposed to face the circumferential side surface of the rotary shaft  1 , an intermediate foil  10  disposed on radially outside of the top foil  9 , a back foil  11  disposed on radially outside of the intermediate foil  10 , and a bearing housing  12  (housing) disposed on radially outside of the back foil  11 . 
     The bearing housing  12  is a cylindrical member that configures the outermost part of the radial foil bearing  3 . The side surface of the bearing housing  12  is provided with the insertion hole  12   a  (a hole causing the bottom surface and the top surface of the cylindrical member to communicate with each other), and the rotary shaft  1  is inserted through the insertion hole  12   a . The back foil  11 , the intermediate foil  10 , and the top foil  9  are arranged in this order radially inward from outside between the bearing housing  12  and the rotary shaft  1 . That is, the back foil  11 , the intermediate foil  10 , and the top foil  9  are accommodated in the insertion hole  12   a  of the bearing housing  12 . Thus, the back foil  11  is supported by an inner circumferential surface of the insertion hole  12   a , the intermediate foil  10  is supported by the back foil  11 , and the top foil  9  is supported by the intermediate foil  10 . The bearing housing  12  may be a member other than a cylindrical member (for example, a square post) as long as the insertion hole  12   a  is provided therein. A groove  14  is formed on the inner circumferential surface of the insertion hole  12   a  of the bearing housing  12  in the axial direction of the bearing housing  12 . That is, the groove  14  is formed on the inner circumferential surface of the bearing housing  12  on the entire length in the axial direction of the bearing housing  12 . In other words, in a cross-section of the bearing housing  12  in a direction orthogonal to the axial direction of the insertion hole  12   a , the inner circumferential surface is provided with a recess that is recessed radially outward. The groove  14  can accommodate ends of the top foil  9 . 
     As shown in  FIG. 2 , a pair of engagement grooves  15  extending radially outward from the inner peripheral edge of the insertion hole  12   a  are formed in both side surfaces (end surfaces in the axial direction) of the bearing housing  12 . The engagement groove  15  of this embodiment is formed at each of positions by which the side surface of the bearing housing  12  is divided into approximately three areas in the circumferential direction. That is, three pairs of the engagement grooves  15  are provided in the bearing housing  12  of this embodiment. Engagement members  30  described later engage with the engagement grooves  15 . In this embodiment, the groove  14  is disposed between two pairs of engagement grooves  15  among the three pairs of the engagement grooves  15 . In addition, one pair of engagement grooves  15  faces the groove  14  in the radial direction. In order to form the engagement grooves  15 , cutting machining with an end mill, electrolytic machining, wire-cut electric discharge machining or the like can be appropriately used. In addition, the engagement groove  15  may not be formed to penetrate from the inner peripheral edge to the outer peripheral edge of the bearing housing  12 . For example, the engagement groove  15  may open only at the inner circumferential surface of the bearing housing  12 . 
     The top foil  9  is cylindrically wound along the inner surface of the intermediate foil  10  and is arranged such that a first uneven portion  23   a  formed in one end of the top foil  9  and a second uneven portion  23   b  formed in another end thereof engage with the groove  14  formed in the bearing housing  12 . As shown in  FIG. 3A  that is a development view of the top foil  9 , the top foil  9  is formed by winding a rectangular metal foil whose long side is in the bearing circumferential direction and whose short side is in the bearing length direction (the axial direction), and the metal foil is cylindrically wound in the arrow direction (the length direction of the long side; the bearing circumferential direction) in  FIG. 3B  that is a side view of the top foil  9 . 
     As shown in  FIG. 3A , the first uneven portion  23   a  including one protruding part  21   a  and two recessed parts  22   a  is formed in one side (short side) of the top foil  9 , and the second uneven portion  23   b  including two protruding parts  21   b  and one recessed part  22   b  is formed in another side (short side) thereof opposite to the one side (short side). The recessed part  22   b  of the second uneven portion  23   b  is formed corresponding to the protruding part  21   a  of the first uneven portion  23   a , and the recessed parts  22   a  of the first uneven portion  23   a  are formed corresponding to the protruding parts  21   b  of the second uneven portion  23   b.    
     That is, the recessed part  22   b  of the second uneven portion  23   b  is formed such that the protruding part  21   a  passes through the recessed part  22   b , when the top foil  9  is cylindrically wound such that the first uneven portion  23   a  and the second uneven portion  23   b  overlap each other. Similarly, the recessed parts  22   a  of the first uneven portion  23   a  are formed such that the protruding parts  21   b  pass through the recessed parts  22   a , when the top foil  9  is cylindrically wound. 
     As shown in  FIG. 2 , the protruding parts  21   a  and  21   b  passed through the recessed parts  22   b  and  22   a , respectively, are pulled out toward the bearing housing  12 , and the end parts of the protruding parts  21   a  and  21   b  are accommodated in the groove  14  of the bearing housing  12  (the ends of the top foil  9  are in contact with and held by inner wall surfaces of the groove  14 ). That is, both ends in the circumferential direction of the top foil  9  are held by an inner circumferential surface of the insertion hole  12   a  of the bearing housing  12 . The top foil  9  is arranged such that the ends thereof contact inner surfaces of the groove  14 . 
     As shown in  FIG. 3B , the top foil  9  is provided with thin portions  24  at a side (the one side) thereof on which the first uneven portion  23   a  is formed and at a side (the other side) thereof on which the second uneven portion  23   b  is formed, and the thin portions  24  are thinner than a central portion of the top foil  9  between the sides. As shown in  FIG. 2 , each of these thin portions  24  is formed to be thin (thinned) such that the outer circumferential surface (the surface close to the bearing housing  12 ) thereof is recessed compared to the outer circumferential surface of the central portion. 
     The thin portion  24  is formed to have a desired thickness (thinness) by controlling the thickness of both ends of the top foil  9  in the order of 10 μm through, for example, etching. Specifically, in a case where the bearing diameter is 35 mm, when the thickness of the top foil  9  is 100 μm, the thickness of the thin portion  24  is about 80 μm. 
     As shown in  FIG. 2 , the length L in the circumferential direction of the thin portion  24  shown in  FIG. 3B  is a length corresponding to the groove  13  and one hill part  11   c  of an end of the back foil  11 . In addition, the length L in the circumferential direction of the thin portion  24  may be a length corresponding to the groove  14  and about three hill parts  11   c  of the end of the back foil  11 , instead of the example shown in  FIG. 2 . 
     As shown in  FIG. 2 , the back foil  11  is disposed on the inner circumferential surface of the insertion hole  12   a  of the bearing housing  12 . The back foil  11  is formed of a foil (thin sheet) and elastically supports the intermediate foil  10  and the top foil  9 . For such a back foil  11 , for example, a bump foil, a spring foil described in Japanese Unexamined Patent Application, First Publication No. 2006-57652, Japanese Unexamined Patent Application, First Publication No. 2004-270904 or the like, or a back foil described in Japanese Unexamined Patent Application, First Publication No. 2009-299748 or the like is used. In this embodiment, a bump foil is used for the back foil  11 . However, the spring foil or the back foil described above may be used for the back foil of the present disclosure. 
     In the present disclosure, the back foil  11  is configured of three (a plurality of) back foil pieces  11   a  disposed in the circumferential direction of the bearing housing  12 . In each of these back foil pieces  11   a , a foil (thin sheet) is formed into a wave sheet shape in the circumferential direction. In addition, the back foil piece is formed such that the side shape thereof viewed in the axial direction becomes a substantially arc shape as a whole. In the present disclosure, all of the three back foil pieces  11   a  are formed to have equal shape and size. Therefore, these back foil pieces  11   a  are arranged to divide the inner circumferential surface of the bearing housing  12  into approximately three areas. The number of the back foil pieces configuring the back foil  11  may be appropriately changed. 
     In addition, the “wave sheet shape” in this embodiment is not limited to a shape configured only of curved surfaces (for example, a sine wave), but denotes a shape in which a radially inward protruding part and a radially outward protruding part are alternately disposed in the circumferential direction. The shape may have a flat portion, namely a linearly extending portion, when viewed in the axial direction and may be configured by combining a plurality of flat portions together. 
     The back foil pieces  11   a  at positions between which the groove  14  is interposed are arranged with a gap. On the other hand, at the other positions, the ends of the back foil pieces  11   a  are arranged to be close to each other (with a gap less than the gap in which the groove  14  is positioned). That is, the back foil piece  11   a  does not extend to the position in the circumferential direction of the groove  14 . By such a configuration, the three back foil pieces  11   a  are formed in a substantially cylindrical shape as a whole and are disposed along the inner circumferential surface of the bearing housing  12 . That is, when the back foil piece  11   a  is viewed in the axial direction, hill parts  11   c  protruding radially inward and valley parts  11   b  protruding radially outward compared to the hill parts  11   c  are alternately formed in the circumferential direction. 
     The valley part of this embodiment includes the flat valley part  11   b  facing the bearing housing  12 . The flat valley part  11   b  can contact the inner circumferential surface of the insertion hole  12   a . In addition, the hill part  11   c  can contact the intermediate foil  10  (an intermediate foil piece  10   a  described later). Therefore, the back foil piece  11   a  elastically supports the top foil  9  via the intermediate foil piece  10   a , particularly using the hill parts  11   c  contacting the intermediate foil piece  10   a  (the intermediate foil  10 ). 
     In addition, fluid passageways in the radial direction of the radial foil bearing  3  are formed by the hill parts  11   c  or the valley parts  11   b . Furthermore, both ends in the circumferential direction of the back foil piece  11   a  of this embodiment are the valley parts. 
       FIG. 5A  is a diagram schematically showing an important part of  FIG. 2  in a flattened manner. A cutout  16  is formed in each of edges of both ends in the axial direction of a portion (the central portion in a direction parallel to the circumferential direction of the bearing housing  12 ) of each back foil piece  11   a  between both ends thereof in the circumferential direction. That is, a position of the edge extending in the circumferential direction of the back foil piece  11   a , which is between both ends thereof in the circumferential direction, is provided with a recess that is recessed toward the central position thereof in the axial direction. As shown in  FIG. 4 , the cutout  16  is formed in the valley part  11   b  of the back foil piece  11   a . The cutout  16  of this embodiment is formed by cutting out an area including the valley part  11   b  and root parts of the hill parts  11   c  adjacent to each other across the valley part  11   b , toward the center in the axial direction of the bearing housing  12 . That is, the cutout  16  is formed in a position extending in the circumferential direction and including the valley part  11   b . The cutout  16  is formed at a position corresponding to the engagement groove  15  of the bearing housing  12 , that is, a position overlapping the engagement groove  15 . The width in the circumferential direction of the cutout  16  is formed to be the same as the width in the circumferential direction of the engagement groove  15 . 
     As shown in  FIG. 2 , the intermediate foil  10  is disposed between the top foil  9  and the back foil  11  configured of the three back foil pieces  11   a . In this embodiment, the intermediate foil  10  is configured of three intermediate foil pieces  10   a  arranged in the circumferential direction of the bearing housing  12 . As shown in  FIG. 5A , the intermediate foil piece  10   a  is formed such that the developed shape thereof becomes a substantially rectangular shape, and is curved at a predetermined curvature such that a substantially cylindrical shape is formed of the three intermediate foil pieces  10   a , thereby having an arc shape in side view. Each intermediate foil piece  10   a  of this embodiment faces the hill parts  11   c  of the back foil piece  11   a  in the radial direction. 
     That is, the intermediate foil piece  10   a  can come into contact with the hill parts  11   c  disposed at positions in the circumferential direction closest to both ends in the circumferential direction of the back foil piece  11   a . Moreover, the separation distance in the circumferential direction between the back foil pieces  11   a  adjacent to each other without the groove  14  disposed therebetween is less than the separation distance in the circumferential direction between the intermediate foils  10  corresponding to the back foil pieces  11   a  and positioned on radially inside of the back foil pieces  11   a.    
     As can be understood from the disclosure to this point, the radial foil bearing  3  of this embodiment includes the top foil  9  formed of one sheet of foil, the back foil  11  formed of three sheets of foil, and the intermediate foil  10  formed of three sheets of foil. The number of the intermediate foil pieces configuring the intermediate foil  10  may be appropriately changed. 
     As shown in  FIGS. 5A and 5B , the intermediate foil piece  10   a  includes flat portions  10   b  that contact the tops of the hill parts  11   c  of the back foil  11 , and a recess  10   c  (protruding part) that is recessed (protrudes) radially outward compared to the flat portions  10   b.    
     The recess  10   c  is formed so as to be recessed radially outward from the radially inner surface of the intermediate foil piece  10   a  and to protrude radially outward from the radially outer surface of the intermediate foil piece  10   a.    
     That is, the recess  10   c  is away from the top foil  9 . As shown in  FIG. 5A , the recess  10   c  is formed at a position in the circumferential direction between both ends in the circumferential direction of the intermediate foil piece  10   a . The recess  10   c  of this embodiment includes a bottom positioned radially outward and being flat in the circumferential direction, and tapered parts positioned on both ends in the circumferential direction of the bottom and extending radially inward. That is, the recess  10   c  extends radially inward from the bottom as it goes away from the bottom in the circumferential direction. The separation in the circumferential direction between the tapered parts of the recess  10   c  gradually decreases radially outward from the radially inner side thereof. In addition, the width in the circumferential direction of the bottom of the recess  10   c  is greater than the width in the circumferential direction of the valley part  11   b  of the back foil piece  11   a . In this embodiment, the valley part  11   b  is formed to be flat. However, if the hill part  11   c  and the valley part  11   b  have one peaks and are periodically (namely, sinusoidally) formed, then the width in the circumferential direction of the valley part  11   b  in the present disclosure is considered to be the dimension of the valley part  11   b  of the back foil piece  11   a  at the position in the radial direction of the middle between the peak of the hill part  11   c  and the peak of the valley part  11   b . The outer shape of the intermediate foil piece  10   a  has substantially the same size as the outer shape of the back foil piece  11   a . All of the three intermediate foil pieces  10   a  are formed to have equal shape and size. Therefore, these intermediate foil pieces  10   a  are disposed to divide the inner circumferential surface of the bearing housing  12  into approximately three areas. 
     The intermediate foil pieces  10   a  are disposed at positions corresponding to the back foil pieces  11   a . The intermediate foil pieces  10   a  at positions between which the groove  14  is interposed are disposed with a gap. The intermediate foil pieces  10   a  at the other positions are disposed such that the ends thereof are close to each other. That is, in this embodiment, the valley parts  11   b  positioned at both ends of the back foil piece  11   a  do not reach the position in the circumferential direction of the groove  14 . The intermediate foil pieces do also not reach the position in the circumferential direction of the groove  14 . The thickness of the intermediate foil piece  10   a  is less than that of the back foil piece  11   a . The rigidity of the intermediate foil  10  is less than or equal to half the rigidity of the back foil  11 . By such a configuration, the three intermediate foil pieces  10   a  are formed in a substantially cylindrical shape as a whole and are disposed to be supported by the back foil  11  along the inner circumferential surface of the bearing housing  12 . 
     As shown in  FIG. 5A , a cutout  17  is formed in each of edges of both ends in the axial direction of a portion (the central portion in a direction parallel to the circumferential direction of the bearing housing  12 ) of each intermediate foil piece  10   a  between both ends thereof in the circumferential direction. That is, a position of the edge extending in the circumferential direction of the intermediate foil piece  10   a , which is between both ends thereof in the circumferential direction, is provided with a recess that is recessed toward the central position in the axial direction. As shown in  FIG. 4 , the cutout  17  is formed in the recess  10   c  of the intermediate foil piece  10   a . The cutout  17  of this embodiment is formed by cutting out part of the bottom of the recess  10   c  formed between the flat portions  10   b , toward the center in the axial direction of the bearing housing  12  from the side edge. 
     The cutout  17  is formed at a position corresponding to the engagement groove  15  of the bearing housing  12  and to the cutout  16  of the back foil piece  11   a , namely, a position overlapping the engagement groove  15  and the cutout  16 , and the width in the circumferential direction of the cutout  17  is formed to be the same as the width in the circumferential direction of each of the engagement groove  15  and the cutout  16 . 
     An engagement member  30  engages with the engagement groove  15  and the cutouts  16  and  17 . As shown in  FIGS. 5A and 5B , the engagement member  30  includes a pair of engagement legs  31  extending radially outward and a connection portion  32  extending in the axial direction, and the connection portion  32  connects the pair of engagement legs  31  to each other. One of the engagement legs  31  engages with the engagement groove  15  and the cutouts  16  and  17  on one side of the radial foil bearing  3 , and the other of the engagement legs  31  engages with the engagement groove  15  and the cutouts  16  and  17  on the other side of the radial foil bearing  3 . That is, the engagement leg  31  is inserted in the engagement groove  15 . 
     As shown in  FIG. 5B , the length in the radial direction of the engagement leg  31  is approximately equal to the sum of the thickness of the bearing housing  12 , the thickness of the back foil piece  11   a , and the thickness of the intermediate foil piece  10   a . The width of the engagement leg  31  is approximately equal to the width of each of the engagement groove  15  and the cutouts  16  and  17 . That is, as shown in  FIG. 4 , the connection portion  32  is disposed inside the valley part  11   b  at the central portion in the circumferential direction of the back foil piece  11   a  and inside the recess  10   c  of the intermediate foil piece  10   a.    
     By such a configuration, since the engagement leg  31  engages with the engagement groove  15  of the bearing housing  12 , the cutout  16  of the back foil piece  11   a , and the cutout  17  of the intermediate foil piece  10   a , the engagement member  30  serves as a holding member (retainer) that holds the intermediate foil piece  10   a  and the back foil piece  11   a  on the bearing housing  12 . In addition, the connection portion  32  of the engagement member  30  is covered with the top foil  9 . In other words, the connection portion  32  is held between the bottom of the recess  10   c  of the intermediate foil  10  and the top foil  9 . 
     The engagement leg  31  and the connection portion  32  of the engagement member  30  may be in the shape of a square pole as shown in  FIG. 4  or may be in the shape of a column (round bar). The thickness of the engagement member  30  is set such that the connection portion  32  is arranged to be separated from the top foil  9  without contacting the top foil  9 . The engagement member  30  can be formed, for example, by etching a metal foil made of stainless steel or the like into a U-shape. It can also be formed by bending a wire-shaped metal rod. 
     As shown in  FIG. 6  that is an enlarged view of an important part shown in  FIG. 5B , in a portion in which the engagement member  30  is disposed, the intermediate foil piece  10   a  and the back foil piece  11   a  can contact each other at positions between which the engagement member  30  is interposed. That is, in the intermediate foil piece  10   a , in addition to a contact point P 1  in which the flat portion  10   b  contacts the peak of the hill part  11   c  of the back foil piece  11   a , a contact point P 2  is formed in which the recess  10   c  contacts a portion other than the peak of the hill part  11   c  of the back foil piece  11   a.    
     At positions between which a bottom surface  11   b   1  is interposed as shown in  FIG. 4 , as shown in  FIG. 6 , the recess  10   c  of the intermediate foil piece  10   a  contacts side surfaces  11   c   1  of the hill parts  11   c  of the back foil piece  11   a  by the engagement member  30 . The side surfaces  11   c   1  are positioned on both sides in the circumferential direction of the peak of the hill part  11   c  and radially outward compared to the peak. That is, the recess  10   c  is disposed at a position in the circumferential direction between the peak of one hill part  11   c  and the peak of a hill part  11   c  adjacent to the one hill part  11   c  in the wave sheet shape of the back foil piece  11   a . In other words, the recess  10   c  is disposed at a position in the circumferential direction corresponding to a portion of the wave sheet shape of the back foil piece  11   a  between the peak of one hill part  11   c  and the peak of a hill part  11   c  adjacent to the one hill part  11   c . The side surface  11   c   1  of the hill part  11   c  is a curved surface, and a side-wall surface  10   c   1  (a contact surface, a radially outer surface of the tapered part) of the recess  10   c  in contact with the side surface  11   c   1  is an inclined surface. The separation in the circumferential direction between a pair of side-wall surfaces  10   c   1  gradually decreases radially outward from the radially inner side thereof. That is, the side surface  11   c   1  of the hill part  11   c  and the side-wall surface  10   c   1  of the recess  10   c  contact each other at one point of the contact point P 2  and are separated from each other at other positions, and energy dissipation easily occurs due to friction at the contact point P 2 . Thus, two contact points P 2  are formed in one engagement member  30 . 
     As shown in  FIG. 6 , the side surface  11   c   1  of the hill part  11   c  is a curved surface bulging radially inward when viewed in the axial direction, but may linearly extend when viewed in the axial direction. The side-wall surface  10   c   1  of the recess  10   c  linearly extends when viewed in the axial direction, but may be a curved surface bulging radially outward when viewed in the axial direction. If at least one of the side surface  11   c   1  and the side-wall surface  10   c   1  is a curved surface bulging toward the other thereof, these surfaces can contact each other at one contact point and be separated from each other at other positions. 
     Next, the operation of the radial foil bearing  3  having such a configuration will be described. 
     In a state where the rotary shaft  1  stops, the top foil  9  is pushed by the back foil  11  (three back foil pieces  11   a ) via the intermediate foil  10  (three intermediate foil pieces  10   a ) toward the rotary shaft  1  and thus closely contacts the rotary shaft  1 . In addition, in this embodiment, since both ends of the top foil  9  are the thin portions  24 , at the thin portions  24 , a force (local preload) that clamps the rotary shaft  1  does almost not occur. 
     Then, when the rotary shaft  1  is started in an arrow P direction in  FIG. 2 , the rotary shaft  1  starts rotating at a low speed at first, and then gradually accelerates to rotate at a high speed. Then, as shown by an arrow Q in  FIG. 2 , ambient fluid is drawn in from one end of each of the top foil  9 , the intermediate foil  10 , and the back foil  11 , and flows into a space between the top foil  9  and the rotary shaft  1 . As a result, a fluid lubrication film is formed between the top foil  9  and the rotary shaft  1 . 
     The film pressure of the fluid lubrication film acts on the top foil  9  and presses each hill part  11   c  of the back foil piece  11   a  via the intermediate foil  10  in contact with the top foil  9 . Then, the back foil piece  11   a  is pressed by the intermediate foil  10  so that the hill part  11   c  is pressed and expanded, whereby the back foil piece  11   a  starts moving in the circumferential direction on the bearing housing  12 . That is, since the back foil piece  11   a  (the back foil  11 ) elastically supports the top foil  9  via the intermediate foil  10 , the back foil piece  11   a  deforms in the circumferential direction when receiving a load from the top foil  9 , and accepts flexure of the top foil  9  or the intermediate foil  10  and supports them. 
     As shown in  FIG. 5B , the engagement leg  31  of the engagement member  30  is inserted in and engaged with the cutout  16  provided on the side circumferential edge part of the back foil piece  11   a  and serves as a rotation stopper with respect to the bearing housing  12 . Thus, each hill part  11   c  of the back foil piece  11   a  deforms (moves) in the circumferential direction in a state where the cutout  16  with which the engagement member  30  is engaged serves as a fixed point (a fixed end), but the center of the back foil piece  11   a  does not move from a fixed position. 
     The back foil piece  11   a  is affected by the friction between the back foil piece  11   a  and the bearing housing  12  or the intermediate foil  10  when deforming (moving) in the circumferential direction, and thus easily deforms (moves) at both ends, namely, free end sides thereof, but does not easily deform at the fixed point (the fixed end) side thereof. 
     Therefore, a difference may occur in the support rigidity of the back foil piece  11   a  between the free end side and the fixed end side thereof. However, in this embodiment, since the cutout  16  is formed in the central portion in the circumferential direction of the back foil piece  11   a , the fixed point by the engagement member  30  is in the central portion in the circumferential direction of the back foil piece  11   a , the distance between the fixed end and the free end decreases, and the difference in the support rigidity decreases. 
     Furthermore, in this embodiment, since the back foil  11  is divided into the three back foil pieces  11   a , the distance between the fixed end and the free end decreases compared to a case where the back foil  11  is configured of a single foil, and thus the difference in the support rigidity between the fixed end side and the free end side thereof further decreases. 
     Even when an unexpected impact or the like acts while the rotary shaft  1  rotates at a high speed, since the engagement member  30  limits the back foil piece  11   a  from moving in the axial direction, the back foil piece  11   a  does not drop off the bearing housing  12 . Similarly, the intermediate foil piece  10   a  is provided with the recess  10   c  in which the connection portion  32  of the engagement member  30  is disposed, the engagement leg  31  of the engagement member  30  is disposed in the cutout  17  formed in the recess  10   c , and the intermediate foil piece  10   a  is engaged with the engagement groove  15  via the engagement member  30 , whereby even when an unexpected impact or the like acts, the intermediate foil piece  10   a  does not rotate inside the bearing housing  12 , and is limited from moving in the axial direction inside the bearing housing  12 . In addition, since the engagement member  30  is covered with the top foil  9  in the radial direction, the engagement member  30  is limited from dropping off the radial foil bearing  3 . As a result, the intermediate foil piece  10   a  is limited from dropping off the radial foil bearing  3 . 
     The intermediate foil  10  is provided with the recess  10   c  in which the connection portion  32  of the engagement member  30  can be disposed, and the recess  10   c  protrudes toward the back foil  11  disposed radially outward, whereby as shown in  FIG. 6 , the recess  10   c  contacts the side surfaces  11   c   1  of the hill parts  11   c  of the back foil  11  at positions between which the bottom surface  11   b   1  of the valley part  11   b  of the back foil  11  is interposed. Therefore, the intermediate foil  10  and the back foil  11  do not contact each other only at the flat portion  10   b  but also contact at the recess  10   c , and the contact area therebetween increases, so that the damping effect due to friction between the intermediate foil  10  and the back foil  11  is improved, and the stability when the rotary shaft  1  rotating at a high speed is supported can be further enhanced. 
     As shown in  FIG. 6 , two contact points P 2  are formed per one engagement member  30 , and in this embodiment, as shown in  FIG. 2 , the two contact points P 2  are formed in each of pairs (three pairs) of the intermediate foil piece  10   a  and the back foil piece  11   a , so that the contact area between the intermediate foil  10  and the back foil  11  can be increased by six contact points P 2  in addition to the contact point P 1 . In addition, the side surface  11   c   1  of the hill part  11   c  of the back foil  11  is a curved surface, the side-wall surface  10   c   1  of the recess  10   c  contacting the side surface  11   c   1  is an inclined surface, so that these surfaces contact at one point of the contact point P 2  and are separated from each other at other positions. Therefore, the intermediate foil  10  and the back foil  11  do not easily bold each other, and “sliding” therebetween easily occurs. 
     Second Embodiment 
     Next, a second embodiment of the radial foil bearing of the present disclosure will be described. In the following description, the same or equal component as or to that of the above-described embodiment is attached with an equal reference sign, and the explanation thereof will be simplified or omitted. 
       FIGS. 7A and 7B  are diagrams showing a radial foil bearing  3 A of the second embodiment applied to the turbo machine shown in  FIG. 1 ,  FIG. 7A  is a diagram schematically showing an important part of the radial foil bearing  3 A in a flattened manner, and  FIG. 7B  is a side view thereof. 
     The second embodiment is different from the above-described embodiment in that an intermediate foil  10 A (an intermediate foil piece  10   a ) is provided with branching parts  40  (protruding part) branched off from a flat portion  10   b . That is, a portion protruding radially outward is provided in a radially outer surface of the intermediate foil piece  10   a.    
     As shown in  FIG. 7A , a slit  41  is formed in the flat portion  10   b , and the portion surrounded by the slit  41  is pushed out (cut and raised) so as to protrude radially outward, whereby the branching part  40  is formed. In this embodiment, the slit  41  is formed of two incisions parallel to the circumferential direction, and one incision connecting ends of the two incisions and parallel to the axial direction. The rectangular branching part  40  surrounded by the slit  41  is slantingly pushed out to protrude radially outward. That is, the branching part  40  extends from the ends of the two parallel incisions not connecting to the incision parallel to the axial direction (the ends on far side from the incision parallel to the axial direction). 
     The branching part  40  separates in the circumferential direction from the above ends not connecting to the incision parallel to the axial direction and separates radially outward. 
     That is, the branching part  40  extends radially outward as it goes in the circumferential direction toward the incision parallel to the axial direction, from the ends of the two parallel incisions on far side from the incision parallel to the axial direction. 
     In other words, the position of the branching part  40  monotonously changes radially outward as it separates in the circumferential direction from the ends not connecting to the incision parallel to the axial direction. That is, the branching part  40  of this embodiment lineally extends when viewed in the axial direction. The branching part  40  protrudes toward the back foil piece  11   a  at a position in the circumferential direction between the peak of at least one hill part  11   c  and the peak of a hill part  11   c  adjacent to the one hill part  11   c  in the wave sheet shape of the back foil piece  11   a.    
     As shown in  FIG. 7B , a pair of branching parts  40  are provided such that the hill part  11   c  of the back foil piece  11   a  is interposed therebetween in the circumferential direction. That is, as shown in  FIG. 7A , the slit  41  is formed on each of both sides in the circumferential direction of a portion corresponding to the hill part  11   c  of the back foil piece  11   a , and a pair of branching parts  40  pushed out from these slits  41  extend away from each other. That is, two pairs of incisions parallel to the circumferential direction are provided to overlap the position in the circumferential direction of one hill part  11   c , and the two pairs are away from each other in the circumferential direction. The separation in the circumferential direction between the pair of branching parts  40  protruding toward one hill part  11   c  gradually increases radially outward from the radially inner side thereof. In addition, the incision parallel to the axial direction extend from the ends on the side apart from each other of the two incisions parallel to the circumferential direction. By this configuration, two branching parts  40  are configured such that the hill part is interposed therebetween, both ends thereof in the circumferential direction protrude radially outward, and a portion between the ends is positioned radially inward. As shown in  FIG. 7B , two branching parts  40  (a pair of branching parts) provided between which the hill part  11   c  of the back foil piece  11   a  is interposed in the circumferential direction are in contact with the hill part  11   c  of the back foil piece  11   a  so as to hold the hill part  11   c  from both sides thereof in the circumferential direction. In this embodiment, as shown in  FIG. 7A , the flat portion  10   b  is provided with the slits  41  in two rows in the circumferential direction, and four branching parts  40  contact one hill part  11   c . In addition, the slits  41  (the branching parts  40 ) may be provided in a single row or in three or more rows in the circumferential direction. 
       FIG. 8  shows an enlarged cross-sectional view taken along line A-A of an important part shown in  FIG. 7A . The flat portion  10   b  of the intermediate foil  10  (a portion positioned on a plane and obtained by tracing an approximately circular closed curve surrounded in the circumferential direction and with no branching, in the axial direction width of the intermediate foil  10 ) is provided with two branching positions  43  from which the branching parts  40  branches off, between the peak of a first hill part  11   c  (the left hill part  11   c  shown in  FIG. 8 ) and the peak of a second hill part  11   c  (the right hill part  11   c  shown in  FIG. 8 ) adjacent to each other in the circumferential direction of the back foil  11 . In addition, in this embodiment, the flat portion  10   b  is provided with a second branching position  43  (the right branching position  43  shown in  FIG. 8 ) at a position different from a branching position  43  (the left branching position  43  shown in  FIG. 8 ), between the peak of the first hill part  11   c  and the peak of the second hill part  11   c . That is, two branching parts  40  are formed between the peak of the first hill part  11   c  and the peak of the second hill part  11   c  adjacent to each other in the circumferential direction of the back foil  11 . In other words, two (a pair of) branching positions  43  are provided in the circumferential direction between the valley parts  11   b  adjacent to each other in the circumferential direction. In the radial foil bearing  3  of this embodiment, in a state where the rotary shaft  1  inserted therethrough does not rotate, the intermediate foil piece  10   a  and the back foil piece  11   a  are separated from each other in the radial direction at a position sifted in the radial direction from the branching position  43 . 
     That is, in the radial foil bearing  3 , in a state where the inserted rotary shaft  1  does not rotate, the intermediate foil piece  10   a  and the back foil piece  11   a  are separated from each other in the radial direction at a position equal in the circumferential direction to the branching position  43 . 
     A pair of branching parts  40 , which open in directions away from each other, are in contact with positions (the side surfaces  11   c   1 ) between which the peak of the hill part  11   c  is interposed in the circumferential direction. That is, the intermediate foil piece  10   a  is provided with contact points P 3  in which the branching parts  40  contact portions other than the peaks of the hill parts  11   c  of the back foil piece  11   a , in addition to the contact points P 1  in which the flat portion  10   b  contacts the peaks of the hill parts  11   c  of the back foil piece  11   a.    
     In addition, in a state where the inserted rotary shaft  1  does not rotate, no contact point may be formed between the two contact points P 3  between which the peak of the hill part  11   c . Even in this case, when the inserted rotary shaft  1  rotates, the contact point P 1  is formed between the two contact points P 3  between which the peak of the hill part  11   c  is interposed. In addition, the radially outer surface of the branching part  40  extends from the peak side of a hill part  11   c  toward a valley part  11   b  adjacent to the hill part  11   c  beyond the contact point P 3 . However, the radially outer surface thereof does not reach the position in the radial direction of the valley part  11   b  adjacent to the hill part  11   c.    
     The side surface  11   c   1  of the hill part  11   c  is a curved surface, and the branching part  40  in contact with the side surface  11   c   1  is flat. That is, the side surface  11   c   1  of the hill part  11   c  and the branching part  40  contact each other at one point of the contact point P 3  and separate from each other at other positions, and thus sliding at the contact point P 3  easily occurs. 
     According to the second embodiment having the above configuration, the branching position  43  and the second branching position  43  at which the branching parts  40  branch off from the flat portion  10   b  are provided between the peak of the first hill part  11   c  and the peak of the second hill part  11   c  adjacent to each other in the circumferential direction of the back foil  11 , and as shown in  FIG. 8 , the branching parts  40  contact positions other than the peak of the hill part  11   c  of the back foil  11 . Therefore, the intermediate foil  10 A and the back foil  11  do not contact each other only at the flat portion  10   b  and the recess  10   c  but also at the branching part  40  Branched off from the flat portion  10   b , and the number of the contact positions therebetween increases, so that the damping effect due to the friction between the intermediate foil  10 A and the back foil  11  is enhanced. That is, two contact points P 3  are further formed per one hill part  11   c.    
     Since a pair of branching parts  40  are provided such that the hill part  11   c  of the back foil  11  is interposed therebetween in the circumferential direction, even when the back foil  11  deforms so as to extend or shrink in the circumferential direction, the contact state with respect to the hill part  11   c  can always be maintained, and the damping effect due to the friction between the intermediate foil  10 A and the back foil  11  can be enhanced. 
     Third Embodiment 
     Next, a third embodiment of the radial foil bearing of the present disclosure will be described. In the following description, the same or equal component as or to that of the above embodiments is attached with an equal reference sign, and the explanation thereof will be simplified or omitted. 
       FIGS. 9A and 9B  are diagrams showing a radial foil bearing  3 B of the third embodiment applied to the turbo machine shown in  FIG. 1 ,  FIG. 9A  is a diagram schematically showing an important part of the radial foil bearing  3 B in a flattened manner, and  FIG. 9B  is a side view thereof. 
     The third embodiment is different from the first and second embodiments in that an intermediate foil  10 B (an intermediate foil piece  10   a ) is provided with a branching part  40 B (a protruding part) branched off from a flat portion  10   b.    
     As shown in  FIG. 9A , the branching part  40 B is formed by forming a slit  41 B in the flat portion  10   b  and pushing out the portion surrounded by the slit  41 B so as to protrude radially outward. That is, the branching part  40 B of this embodiment protrudes radially outward from the flat portion  10   b  in the circumferential direction and further extends radially inward beyond the peak of protruding. In this embodiment, the slit  41 B is formed in an H-shape, and two branching parts  40 B formed by the slit  41 B bulge in a curved shape so as to protrude radially outward, thereby forming a wave shape. That is, when viewed in the radial direction (refer to  FIG. 9A ), the branching parts  40 B extend from the sides at both ends in the circumferential direction of a rectangular area in which the H-shaped slit  41 B is disposed, toward the sides at the ends opposite thereto. The tips of the two branching parts  40 B separates from each other in the circumferential direction. 
     As shown in  FIG. 9B , the branching part  40 B is disposed at a position corresponding to the valley part  11   b  of the back foil piece  11   a . That is, the branching part  40 B includes a position facing the valley part  11   b  in the radial direction and extends to each of the hill parts  11   c  adjacent to both sides of the valley part  11   b  in the circumferential direction. However, the tip of the branching part  40 B is positioned to be closer to the valley part  11   b  than the peak of the hill part  11   c . Therefore, the branching part  40 B extends from one hill part  11   c  to a position in the circumferential direction of a hill part  11   c  adjacent to the one hill part  11   c . As shown in  FIG. 9A , the central portion of the slit  41 B has an area in which the back foil  11  is exposed when viewed in a radial direction. Through the exposing area, the position corresponding to the peak of the hill part  11   c  of the back foil piece  11   a  is exposed. The pair of branching parts  40 B cut and raised from the slit  41 B are deformed in curved surfaces so as to separate from each other, and as shown in  FIG. 9B , the branching parts  40 B contact hill parts  11   c  disposed on both sides of a hill part  11   c  disposed at a position corresponding to the center of the slit  41 B. That is, a hill part  11   c  held from both sides thereof in the circumferential direction by the branching parts  40 B and a hill part  11   c  not held from both sides thereof in the circumferential direction by the branching parts  40 B are alternately arranged in the circumferential direction. In other words, the peak of the hill part  11   c  of the back foil piece  11   a  and the peak of the branching part  40 B alternately appear in the circumferential direction. In further other words, peaks of one branching part  40 B are provided between the top of one hill part  11   c  and the tops of hill parts  11   c  adjacent to the one hill part  11   c . In addition, the “peak” described above denotes a peak of a radial direction position, namely, a portion that protrudes radially outward. 
     As shown in  FIG. 10  that is an enlarged cross-sectional view taken along line B-B of an important part shown in  FIG. 9A , the flat portion  10   b  is provided with a branching position  43 B from which the branching part  40 B branches off, between the top of a first hill part  11   c  and the top of a second hill part  11   c  adjacent to each other in the circumferential direction of the back foil  11 . That is, one branching part  40 B is formed between the top of the first hill part  11   c  and the top of the second hill part  11   c  adjacent to each other in the circumferential direction of the back foil  11 . The branching part  40 B includes a separating part  40 B 1  that separates radially outward from the flat portion  10   b , an approaching part  40 B 2  that extends from the separating part  40 B 1  (the radially outer end of the separating part  40 B 1 ) and approaches radially inward and toward the flat portion  10   b , and an end part  40 B 3  that extends from the approaching part  40 B 2  (the radially inner end of the approaching part  40 B 2 ) along the flat portion  10   b . The separating part  40 B 1  and the approaching part  40 B 2  have smoothly curved shapes with an equal radius of curvature. That is, the separating part  40 B 1  and the approaching part  40 B 2  are formed in a curved surface bulging radially outward as a whole. In addition, each of the separating part  40 B 1  and the approaching part  40 B 2  may be configured by combining a linear shape and another linear shape or by combining a linear shape and a curved shape, when viewed in the axial direction. 
     The separating part  40 B 1  of the branching part  40 B is in contact with the side surface  11   c   1  of the hill part  11   c . That is, the intermediate foil piece  10   a  is provided with a contact point P 4  at which the separating part  40 B 1  of the branching part  40 B contacts a portion other than the top of the hill part  11   c  of the back foil piece  11   a , in addition to the contact point P 1  at which the flat portion  10   b  contacts the top of the hill part  11   c  of the back foil piece  11   a . The side surface  11   c   1  of the hill part  11   c  is a curved surface, and the separating part  40 B 1  of the branching part  40 B in contact with the side surface  11   c   1  is a curved surface that is convex toward the side surface  11   c   1 . That is, the side surface  11   c   1  of the hill part  11   c  and the separating part  40 B 1  of the branching part  40 B contact each other at one point of the contact point P 4  and separate from each other at other positions, and thus sliding at the contact point P 4  easily occurs. In other words, in this embodiment, contact points P 4  are formed by two branching parts  40 B between which the top of one hill part  11   c  is interposed in the circumferential direction. On the other hand, a hill part  11   c  adjacent to the above hill part  11   c  has no contact point with respect to the branching part  40 B. In addition, the length from the contact point P 4  to the tip of the branching part  40 B is greater than the length between the branching position  43 B of the branching part  40 B and the contact point P 4 . That is, in this embodiment, the length from the contact point P 4  to the tip of the branching part  40 B is greater than the length to the contact point P 4  from the top of a hill part  11 P interposed between the contact points P 4 . 
     According to the third embodiment having the above configuration, the branching position  43 B at which the branching part  40 B branches off from the flat portion  10   b  is provided between the top of the first hill part  11   c  and the top of the second hill part  11   c  adjacent to each other in the circumferential direction of the back foil  11 , and as shown in  FIG. 10 , the branching part  40 B is in contact with a position other than the top of the hill part  11   c  of the back foil  11 . Therefore, the intermediate foil  10 B and the back foil  11  do not contact each other only at the flat portion  10   b  and the recess  10   c  but also at the branching part  40 B branched off from the flat portion  10   b , and the number of the contact positions therebetween increases, so that the damping effect due to the friction between the intermediate foil  10 B and the back foil  11  is enhanced. 
     Since a pair of branching parts  40 B are provided such that every other hill part  11   c  is interposed, even when the back foil  11  deforms so as to extend or shrink in the circumferential direction, the branching parts  40 B can contact the hill part  11   c . In addition, the branching part  40 B has a wave sheet shape including the separating part  40 B 1  that separates radially outward from the flat portion  10   b , and the approaching part  40 B 2  that extends from the separating part  40 B 1  and approaches radially inward and toward the flat portion  10   b , and when the back foil  11  deforms in the circumferential direction, the branching part  40 B is pushed by the hill part  11   c  and causes sliding (namely, extends in the circumferential direction) as shown by a reference sign S in  FIG. 10 , similar to the back foil  11  (a bump foil). 
     At this time, “sliding” can also occur between the end part  40 B 3  of the branching part  40 B and the top foil  9 , and the damping effect due to friction is further enhanced. 
     Hereinbefore, although the embodiments of the present disclosure have been described with reference to the attached drawings, the present disclosure is not limited to the above embodiments. The shapes, combinations, and the like of the components described in the above embodiments are merely examples, and addition, omission, replacement, and other modifications of the configuration can be adopted based on design requirements and the like within the scope of the present disclosure. 
     For example, the configuration and number of grooves and retainers for holding the top foil  9 , the intermediate foil  10 , and the back foil  11  are not limited to the above embodiments, and various configurations may be adopted therefor. 
     Although the bearing housing  12  is formed in a cylindrical shape in the above embodiments, one side surface or both side surfaces of the bearing housing may be provided integrally with an annular flange, thereby forming the housing in an approximately cylindrical shape as a whole. By forming the flange, the bearing can be easily attached to a housing or the like of a turbo machine.