Patent Publication Number: US-11028720-B2

Title: Turbine ring assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of PCT/FR2018/050589, filed Mar. 13, 2018, which in turn claims priority to French patent application number 1752151 filed Mar. 16, 2017. The content of these applications are incorporated herein by reference in their entireties. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a turbine ring assembly comprising a plurality of ring sectors made of ceramic-matrix composite material as well as a ring support structure. 
     The field of application of the invention is in particular that of the aeronautical gas turbine engines. The invention is however applicable to other turbomachines, for example industrial turbines. 
     In the case of entirely metallic turbine ring assemblies, it is necessary to cool all the elements of the assembly and particularly the turbine ring which is subjected to the hottest flows. This cooling has a significant impact on the engine performance since the cooling flow used is taken from the main flow of the engine. In addition, the use of metal for the turbine ring limits the possibilities to increase the temperature at the turbine, which would however allow improving the performance of the aeronautical engines. 
     In order to solve these problems, it has been envisaged to produce turbine ring sectors made of ceramic-matrix composite material (CMC) in order to overcome the implementation of a metal material. 
     CMC materials have good mechanical properties making them capable of forming structural elements and advantageously preserve these properties at high temperatures. The implementation of CMC materials has advantageously allowed reducing the cooling flow to be imposed during the operation and therefore increasing the performance of the turbomachines. In addition, the implementation of CMC materials advantageously allows decreasing the weight of the turbomachines and reducing the effect of hot expansion encountered with the metal parts. 
     However, the existing solutions proposed can implement an assembling of a CMC ring sector with metal attachment portions of a ring support structure, these attachment portions being subjected to the hot flow. Consequently, these metal attachment portions undergo hot expansions, which can lead to mechanical stressing of the CMC ring sectors and to an embrittlement thereof. 
     Furthermore, documents FR 2 540 939, GB 2 480 766, EP 1 350 927, US 2014/0271145, US 2012/082540 and FR 2 955 898 which disclose turbine ring assemblies, are known. 
     There is a need to improve existing turbine ring assemblies and their mounting, and in particular the existing turbine ring assemblies implementing a CMC material in order to reduce the intensity of the mechanical stresses to which the CMC ring sectors are subjected during the operation of the turbine. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The invention aims at proposing a turbine ring assembly allowing to maintain each ring sector in a deterministic manner, that is to say, so as to control its position and prevent it from vibrating, on the one hand, while allowing the ring sector, and by extension the ring, to deform under the effects of temperature rises and pressure variations, and this in particular independently of the interface metal parts and, on the other hand, while improving the sealing between the off-flowpath sector and the flowpath sector and while simplifying the manipulations and reducing their number for the mounting of the ring assembly. 
     An object of the invention proposes a turbine ring assembly comprising a plurality of ring sectors forming a turbine ring and a ring support structure, each ring sector having, according to a section plane defined by an axial direction and a radial direction of the turbine ring, a portion forming an annular base with, in the radial direction of the turbine ring, an inner face defining the inner face of the turbine ring and an outer face from which a first and a second attachment tabs protrude, the ring support structure including a central shroud from which a first and a second radial clamps protrude between which the first and second attachment tabs of each ring sector are maintained. 
     According to a general characteristic of the object, the turbine ring assembly comprises a one-piece annular flange removably fastened to the central shroud, the annular flange including a first free end, a second end coupled to the central shroud, a first portion extending from the first end, a second portion extending between the first portion and the second end, the first portion of the flange including a first and a second distinct tabs, the first tab bearing against the first attachment tab and the second tab being spaced apart from the first tab in the axial direction, the second tab being upstream of the first tab relative to the direction of an air flow intended to pass through the turbine ring assembly, and the second portion of the annular flange comprising a bearing shroud protruding downstream in the axial direction, the bearing shroud having a radial bearing in contact with the central shroud of the ring support structure. 
     In a particular embodiment, the ring sectors may be made of ceramic-matrix composite material (CMC). 
     The presence on the first portion of the annular flange of a second tab disposed upstream and separated from a first tab in contact with an upstream attachment tab of the ring allows providing the turbine ring assembly with an upstream tab of the annular flange dedicated to take up the force of the high-pressure distributor (DHP). The second tab upstream of the first tab of the turbine ring and free from any contact with the ring is configured to transit the maximum axial force induced by the DHP directly into the ring support structure without passing through the ring which, when it is made of CMC, has a low mechanical permissible element. 
     Indeed, leaving a space between the first and second tabs of the annular flange allows deflecting the force received by the second tab, upstream of the first tab which is in contact with the turbine ring, and transiting it directly toward the central shroud of the ring support structure via the second portion of the annular flange, without impacting the first tab of the annular flange and therefore without impacting the turbine ring. The first tab of the annular flange do not undergo a force, the turbine ring is thus preserved from this axial force. 
     The transit of the DHP force via the second tab of the annular flange can induce its tilting. This tilting can cause an uncontrolled contact between the low portions that is to say between the tabs, of the annular flange, which would have the consequence of directly transmitting the DHP force to the ring. 
     The downstream bearing shroud ensures higher resistance to the tilting induced by the DHP force. The bearing shroud takes up the significant tangential stresses caused by the DHP force on the upstream tab and thus limits the tilting of the annular flange. The radial bearing of the bearing shroud allows limiting the tilting of the annular flange when the DHP force transits in the flange. 
     In addition, the removable nature of the annular flange makes it possible to have axial access to the cavity of the turbine ring. This allows assembling the ring sectors together outside the ring support structure and then axially sliding the assembly thus assembled into the cavity of the ring support structure until bearing against the second radial clamp, before fastening the annular flange on the central shroud of the ring support structure. 
     During the operation of fastening the turbine ring on the support structure of the ring, it is possible to use a tool including a cylinder or a ring on which the ring sectors are pressed or sucked during their crown assembling. 
     The fact of having an annular flange in one piece, that is to say describing the entirety of a ring over 360°, allows, compared to a sectored annular flange, limiting the passage of the air flow between the off-flowpath sector and the flowpath sector, in so far as all the inter-sector leaks are eliminated, and therefore controlling the sealing. 
     The solution defined above for the ring assembly thus makes it possible to maintain each ring sector in a deterministic manner, that is to say to control its position and prevent it from starting to vibrate, while improving the sealing between the off-flowpath sector and the flowpath sector, while simplifying the manipulations and while reducing their number for the mounting of the ring assembly, and while allowing the ring to deform under the effect of temperature and pressure in particular independently of the interface metal parts. 
     According to a first aspect of the turbine ring assembly, the first radial annular clamp forms a first rib protruding in the radial direction of the turbine ring towards the inside of the ring, and the second end of the annular flange includes an axial abutment extending in the radial direction of the turbine ring towards the outside of the ring, the axial abutment being disposed upstream of said first radial annular clamp and bearing in the axial direction of the turbine ring against said first radial annular clamp. 
     The axial abutment allows pressing the annular flange onto the first radial annular clamp and thus axially positioning the first tab of the annular flange with respect to the upstream radial attachment tab of the ring. 
     According to a second aspect of the turbine ring assembly, the central shroud of the ring support structure may further comprise a second rib protruding in the radial direction of the turbine ring towards the inside of the ring and having a bearing surface on which the radial bearing of the bearing shroud bears, the second rib being disposed between the first and the second radial clamps of the ring support structure. 
     The second rib is a radial bearing point which allows the ring support structure to retain the rocker of the second tab of the annular flange when the DHP force is applied. The large distance between the axial abutment and the radial abutment of the bearing shroud allows increasing the lever arm and thus inducing a smaller radial force on the casing at contact of the radial bearing with the second rib of the ring support structure. 
     The annular flange is fastened by means of two radial shrink-fittings, a first shrink-fitting between the radial bearing and the second rib, and a second shrink-fitting between the surface of the axial abutment extending in a plane comprising the axial direction and the central shroud. 
     According to a third aspect of the turbine ring assembly, the ring sector may have an inverted Greek letter section pi ( 7 ) along the section plane defined by the axial direction and the radial direction, and the assembly may comprise, for each ring sector, at least three pins to radially hold the ring sector in position, the first and second attachment tabs of each ring sector each comprising a first end secured to the outer face of the annular base, a second free end, at least three lugs for receiving said at least three pins, at least two lugs protruding from the second end of one of the first or second attachment tabs in the radial direction of the turbine ring and at least one lug protruding from the second end of the other attachment tab in the radial direction of the turbine ring, each receiving lug including an orifice for receiving one of the pins. 
     According to a forth aspect of the turbine ring assembly, the ring sector may have a section with an elongated K-shape along the section plane defined by the axial direction and the radial direction, the first and a second attachment tabs having an S-shape. 
     According to a fifth aspect of the turbine ring assembly, the ring sector may have, on at least one radial range of the ring sector, an O-section along the section plane defined by the axial direction and the radial direction, the first and second attachment tabs each having a first end secured to the outer face and a second free end, and each ring sector comprising a third and a fourth attachment tabs each extending, in the axial direction of the turbine ring, between a second end of the first attachment tab and a second end of the second attachment tab, each ring sector being fastened to the ring support structure by a fastening screw including a screw head bearing against the ring support structure and a thread cooperating with a tapping formed in a fastening plate, the fastening plate cooperating with the third and fourth attachment tabs. 
     Another object of the invention proposes a turbomachine comprising a turbine ring assembly as defined above. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood upon reading the following, by way of indication but without limitation, with reference to the appended drawings in which: 
         FIG. 1  is a schematic perspective view of a first embodiment of a turbine ring assembly according to the invention; 
         FIG. 2  is an exploded schematic perspective view of the turbine ring assembly of  FIG. 1 ; 
         FIG. 3  is a schematic sectional view of the turbine ring assembly of  FIG. 1 ; 
         FIG. 4  is a schematic sectional view of a second embodiment of the turbine ring assembly; 
         FIG. 5  is a schematic sectional view of a third embodiment of the turbine ring assembly; 
         FIG. 6  is a schematic sectional view of a fourth embodiment of the turbine ring assembly. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a high-pressure turbine ring assembly comprising a turbine ring  1  made of ceramic-matrix composite material (CMC) and a metal ring support structure  3 . The turbine ring  1  surrounds an assembly of rotary blades (not represented). The turbine ring  1  is formed of a plurality of ring sectors  10 ,  FIG. 1  being a radial sectional view. The arrow D A  indicates the axial direction of the turbine ring  1  while the arrow D R  indicates the radial direction of the turbine ring  1 . For reasons of simplification of presentation,  FIG. 1  is a partial view of the turbine ring  1  which is actually a complete ring. 
     As illustrated in  FIGS. 2 and 3 , which respectively have an exploded schematic perspective view and a sectional view of the turbine ring assembly of  FIG. 1 , the sectional view being along a section plane comprising the radial direction D R  and the axial direction D A , each ring sector  10  has, along a plane defined by the axial D A  and radial D R  directions, a section with substantially the shape of the inverted Greek letter (π). The section comprises indeed an annular base  12  and upstream and downstream radial attachment tabs, respectively  14  and  16 . The terms “upstream” and “downstream” are used here with reference to the flowing direction of the gas flow in the turbine represented by the arrow F in  FIG. 1 . The tabs of the ring sector  10  could have another shape, the section of the ring sector having a shape other than π, such as a K- or an O-shape. 
     The annular base  12  includes, along the radial direction D R  of the ring  1 , an inner face  12   a  and an outer face  12   b  opposite to each other. The inner face  12   a  of the annular base  12  is coated with a layer  13  of abradable material forming a thermal and environmental barrier and defines a flow path of gas flow in the turbine. The terms “inner” and “outer” are used herein with reference to the radial direction D R  in the turbine. 
     The upstream and downstream radial attachment tabs  14  and  16  protrude, along the direction D R , from the outer face  12   b  of the annular base  12  away from the upstream and downstream ends  121  and  122  of the annular base  12 . The upstream and downstream radial attachment tabs  14  and  16  extend over the entire width of the ring sector  10 , that is to say, over the entire arc of circle described by the ring sector  10 , or over the entire circumferential length of the ring sector  10 . 
     As illustrated in  FIGS. 1 to 3 , the ring support structure  3  which is secured to a turbine casing comprises a central shroud  31 , extending in the axial direction D A , and having an axis of revolution coincident with the axis of revolution of the turbine ring  1  when they are fastened together, as well as a first radial annular clamp  32  and a second radial annular clamp  36 , the first radial annular clamp  32  being positioned upstream of the second radial annular clamp  36  which is therefore downstream of the first radial annular clamp  32 . 
     The second radial annular clamp  36  extends in the circumferential direction of the ring  1  and, along the radial direction D R , from the central shroud  31  towards the center of the ring  1 . It comprises a first free end  361  and a second end  362  secured to the central shroud  31 . The second radial annular clamp  36  includes a first portion  363 , a second portion  364 , and a third portion  365  comprised between the first portion  363  and the second portion  364 . The first portion  363  extends between the first end  361  and the third portion  365 , and the second portion  364  extends between the third portion  365  and the second end  362 . The first portion  363  of the second radial annular clamp  36  is in contact with the downstream radial attachment clamp  16 . The second portion  364  is thinned relative to the first portion  363  and the third portion  365  so as to give some flexibility to the second radial annular clamp  36  and thus not to stress too much the CMC turbine ring  1 . 
     The first radial annular clamp  32  forms a first radial annular rib extending in the circumferential direction of the ring  1  as well as in the radial direction D R  of the ring, from the central shroud  31  to the center of the ring  1 . 
     As illustrated in  FIGS. 1 to 3 , the turbine ring assembly  1  comprises a unique removable annular flange  35  made in one piece and removably fastened to the ring support structure  3 . The removable flange  35  comprises a first free end  351  and a second end  352  radially shrink-fitted to the central shroud  31  of the annular support structure  3 . The removable flange  35  further comprises a first portion  353  extending from the first end  351  and a second portion  354  extending between the first portion  353  and the second end  352 . 
     The first portion  353  comprises a first tab  33  and a second tab  34  distinct from the first tab  33  and distant from the latter in the axial direction D A , the second tab  34  being upstream of the first tab  33  relative to the direction of the air flow F intended to pass through the turbine ring assembly  1 . When the ring assembly is mounted, the first tab  33  of the removable flange  35  bears against the upstream radial attachment tab  14  of each of the ring sectors  10  forming the turbine ring  1 . 
     The radial holding of the ring  1  is ensured by the first tab  33  of the annular flange  35  which is pressed on the upstream radial attachment tab  14  and by the first portion  363  of the second radial annular clamp  36  which is pressed against the first downstream radial attachment clamp  16 . The first tab  33  of the annular flange  35  ensures the sealing between the flowpath cavity and the off-flowpath cavity of the ring. 
     The second tab  34  of the removable annular flange  35  is dedicated to take up the force of the high-pressure distributor (DHP) on the removable annular flange  35 , on the one hand, by deforming and, on the other hand, by transiting this force towards the casing line which is more mechanically robust, that is to say toward the line of the ring support structure  3  as illustrated by the force arrows E represented in  FIG. 3 . 
     The first tab  33  and the second tab  34  of the removable annular flange  35  meet at the second portion  354  of the removable annular flange  35 . 
     In the first embodiment illustrated in  FIGS. 1 to 3 , the annular flange  35  comprises an axial abutment  355  extending in the radial direction D R  from the second end  352  of the annular flange  35 . The axial abutment  355  extends from the second end  352  towards the central shroud  31  of the ring support structure  3 . The axial abutment  355  is fastened by shrink-fitting on the central shroud  31 . 
     The axial abutment  355  is disposed upstream of the first radial rib formed by the first radial annular clamp  32 , the latter is therefore downstream of the axial abutment  355 . The axial abutment  355  has an upstream face  355   a  receiving the gas flow F and a downstream face  355   b  opposite to the upstream face  355   a  and facing the first radial rib  312 . The first radial rib  32 , that is to say the first radial annular clamp, has an upstream face  32   a  facing the axial abutment  355  of the annular flange  35  and a downstream face  32   b  opposite to the upstream face  32   a  and facing the second radial annular clamp  36 . When the turbine ring assembly is mounted, the downstream face  355   b  of the axial abutment  355  is bearing against the upstream face  32   a  of the first radial rib  32  of the central shroud  31  of the ring structure. 
     The axial abutment  355  has two uses. It allows, on the one hand, the axial positioning of the annular flange  35 , which allows accurately adjusting the axial position of the first tab  33  with respect to the upstream radial attachment tab  14  of the ring, to ensure controlled axial contact between the two parts. The axial abutment  355  allows, on the other hand, limiting the tilting of the second tab  34  and transiting the DHP force axially on the central shroud  31  of the ring support structure  3 . 
     Further, the second end  352  of the annular flange  35  comprises a bearing shroud  356  protruding downstream in the axial direction D A . In other words, the annular flange  35  has an upstream face  35   a  receiving the gas flow F and a downstream face  35   b  opposite to the upstream face  35   a  and facing the first radial annular clamp  32  and the upstream radial attachment tab  14 . The second portion  354  of the annular flange  35  comprises a bearing shroud  356  extending in the axial direction D A  from the downstream face  35   b  of the annular flange  35 . 
     The bearing shroud  356  has an inner face  356   a  and an outer face  356   b  opposite to the inner face  356   a , a first free end  3561 , and a second end  3562  secured to the downstream face  35   b  of the annular flange  35 , the first end  3561  being downstream of the second end  3562  when the turbine ring assembly is mounted. The bearing shroud  356  comprises, on its first end  3561 , a radial bearing  358  protruding from the outer face  356   b  of the bearing shroud  356 . 
     In the embodiment illustrated in  FIGS. 1 to 3 , the central shroud  31  of the ring support structure  3  further comprises a second radial rib  314  disposed between the first radial annular clamp  32  and the second radial annular clamp  36  and protruding in the radial direction D R  from the central shroud  31 . The second radial rib  314  extends towards the ring  1  that is to say towards the radial bearing  358  of the bearing shroud  356 . The second radial rib  314  has at its free end an inner radial face  314   a  facing the radial bearing  358 . The radial bearing  358  has, on its free end, an outer radial face  358   b  facing the second radial rib  314  of the central shroud  31  of the ring support structure  3 . 
     When the turbine ring assembly is mounted, the outer radial face  358   b  of the radial bearing  358  bears against the inner radial face  314   a  of the second radial rib  314 . 
     The bearing shroud  356  ensures higher resistance to the tilting induced by the DHP force. The bearing shroud  356  takes up the significant tangential stresses caused by the DHP force and thus limits the tilting of the annular flange  36 . 
       FIG. 4  represents a sectional view of a second embodiment of the turbine ring assembly. 
     The second embodiment illustrated in  FIG. 4  differs from the first embodiment illustrated in  FIGS. 1 to 3  in that the ring sector  10  has, in the plane defined by the axial D A  and radial D R  directions, a K-shaped section instead of an inverted π-shaped section. 
       FIGS. 5 and 6  represent respectively a schematic sectional view of a third embodiment of the turbine ring assembly and a schematic sectional view of a forth embodiment of the turbine ring assembly. 
     The third and forth embodiments illustrated in  FIGS. 5 and 6  differ from the first embodiment illustrated in  FIGS. 1 to 3  in that the ring sector  10  has, in the plane defined by the axial D A  and radial RD directions, on a portion of the ring sector  10 , an O-shaped section instead of an inverted π-shaped section, the ring section  10  being fastened to the ring support structure  3  by means of a screw  19  and a fastener  20 , the screws  38  being removed. 
     In each of the embodiments of the invention illustrated in  FIGS. 1 to 6 , in the axial direction D A , the second radial annular clamp  36  of the ring support structure  3  is separated from the first tab  33  of the annular flange  35  by a distance corresponding to the spacing of the upstream and downstream radial attachment tabs  14  and  16  so as to maintain these between the first tab  33  of the annular flange  35  and the second radial annular clamp  36 . 
     In the first embodiment illustrated in  FIGS. 1 to 3 , in order to hold the ring sectors  10 , and therefore the turbine ring  1 , in position, with the ring support structure  3 , the ring assembly comprises two first pins  119  cooperating with the upstream attachment tab  14  and the first tab  33  of the annular flange  35 , and two second pins  120  cooperating with the downstream attachment tab  16  and the second radial annular clamp  36 . 
     In the first embodiment, for each corresponding ring sector  10 , the second portion  354  of the annular flange  35  comprises two orifices  3540  for receiving the two first pins  119 , and the third portion  365  of the radial annular clamp  36  comprises two orifices  3650  configured to receive the two second pins  120 . 
     For each ring sector  10 , each of the upstream and downstream radial attachment tabs  14  and  16  comprises a first end  141  and  161  secured to the outer face  12   b  of the annular base  12  and a second free end  142  and  162 . The second end  142  of the upstream radial attachment tab  14  comprises two first lugs  17  each including an orifice  170  configured to receive a first pin  119 . Similarly, the second end  162  of the downstream radial attachment tab  16  comprises two second lugs  18  each including an orifice  180  configured to receive a second pin  120 . The first and second lugs  17  and  18  protrude in the radial direction D R  of the turbine ring  1  respectively from the second end  142  of the upstream radial attachment tab  14  and from second end  162  of the downstream radial attachment tab  16 . 
     The orifices  170  and  180  may be circular or oblong. Preferably, all the orifices  170  and  180  comprise a portion of circular orifices and a portion of oblong orifices. The circular orifices make it possible to tangentially index the rings and to prevent them from moving tangentially (in particular in the event of contact by the vane). The oblong orifices allow accommodating the differential expansions between the CMC and the metal. The CMC has a coefficient of expansion much lower than that of the metal. At high temperature, the lengths in the tangential direction of the ring sector and of the casing portion vis-à-vis each other will therefore be different. If there were only circular orifices, the metal casing would impose its displacements to the CMC ring, which would be a source of very high mechanical stresses in the ring sector. Having oblong holes in the ring assembly allows the pin to slide into this hole and to avoid the overstress phenomenon mentioned above. Therefore, two drilling patterns can be imagined: a first drilling pattern, for a case with three lugs, would comprise a radial circular orifice on a radial attachment clamp and two tangential oblong orifices on the other radial attachment clamp, and a second drilling pattern, for a case with at least four lugs, would comprise a circular orifice and an oblong orifice by radial attachment clamp vis-à-vis each other each time. Other appended cases may be considered as well. 
     For each ring sector  10 , the two first lugs  17  are positioned at two different angular positions with respect to the axis of revolution of the turbine ring  1 . Likewise, for each ring sector  10 , the two seconds lugs  18  are positioned at two different angular positions with respect to the axis of revolution of the turbine ring  1 . 
     As illustrated in  FIG. 4 , in the second embodiment, each ring sector  10  has, along a plane defined by the axial D A  and radial D R  directions, a substantially K-shaped section comprising an annular base  12  with, along the radial direction D R  of the ring, an inner face  12   a  coated with a layer  13  of abradable material forming a thermal and environmental barrier and which defines the flow path of gas flow in the turbine. Substantially S-shaped upstream and downstream radial attachment tabs  140 ,  160  extend, along the radial direction D R , from the outer face  12   b  of the annular base  12  over the entire width thereof and above the upstream and downstream circumferential end portions  121  and  122  of the annular base  12 . 
     The radial attachment tabs  140  and  160  have a first end, referenced respectively  1410  and  1610 , secured to the annular base  12  and a second free end, referenced respectively  1420  and  1620 . The free ends  1420  and  1620  of the upstream and downstream radial attachment tabs  140  and  160  extend either parallel to the plane in which the annular base  12  extends, that is to say along a circular plane, or rectilinearly while the attachment tabs  140  and  160  extend annularly. In this second configuration where the ends are rectilinear and the annular attachment tabs, in the case of a possible swing of the ring during the operation, the surface bearings then become linear bearings thereby providing a greater sealing than in the case of ad hoc bearings. The second end  1620  of the downstream radial attachment tab  160  is held between a portion  3610  of the second radial annular clamp  36  protruding in the axial direction D A  from the first end  361  of the second radial annular clamp  36  in the opposite direction to the flow F direction and the free end of the associated screw  38 , that is to say the screw opposite to the screw head. The second end  1410  of the upstream radial attachment tab  140  is held between a portion  3310  of the first tab  33  of the annular flange  35  protruding in the axial direction D A  from the first end  331  of the first tab  33  in the flow F direction and the free end of the associated screw  38 . 
     In the third embodiment illustrated in  FIG. 5 , the ring sector  10  comprises an axial attachment tab  17 ′ extending between the upstream and downstream radial attachment tabs  14  and  16 . The axial attachment tab  17 ′ extends more precisely, in the axial direction D A , between the second end  142  of the upstream radial attachment tab  14  and the second end  162  of the downstream radial attachment tab  16 . 
     The axial attachment tab  17 ′ comprises an upstream end  171 ′ and a downstream end  172 ′ separated by a central portion  170 ′. The upstream and downstream ends  171 ′ and  172 ′ of the axial attachment tab  17 ′ protrude, in the radial direction D R , from the second end  142 ,  162  of the radial attachment tab  14 ,  16  to which they are coupled, so as to have a central portion  170 ′ of axial attachment tab  17 ′ raised relative to the second ends  142  and  162  of the upstream and downstream radial attachment tabs  14  and  16 . 
     For each ring sector  10 , the turbine ring assembly comprises a screw  19  and a fastener  20 . The fastener  20  is fastened on the axial attachment tab  17 ′. 
     The fastener  20  further comprises an orifice  21  equipped with a tapping cooperating with a thread of the screw  19  to fasten the fastener  20  to the screw  19 . The screw  19  comprises a screw head  190  whose diameter is greater than the diameter of an orifice  39  made in the central shroud  31  of the support structure of the ring  3  through which the screw  19  is inserted before being screwed to the fastener  20 . 
     The bearing shroud  356  further comprises an orifice  3560  traversed by the screw  19  and by a portion of the fastener  20 . The orifice  3560  has a diameter greater than that of the fastener  20 . 
     The radial securing of the ring sector  10  with the ring support structure  3  is carried out using the screw  19 , whose head  190  bears on the central crown  31  of the ring support structure  3 , and the fastener  20  screwed to the screw  19  and fastened to the axial attachment tab  17 ′ of the ring sector  10 , the screw head  190  and the fastener  20  exerting forces of opposite directions in order to hold together the ring  1  and the ring support structure  3 . 
       FIG. 6  represents a schematic sectional view of a fourth embodiment of the turbine ring assembly. 
     The fourth embodiment illustrated in  FIG. 6  is a variant of the third embodiment illustrated in  FIG. 5 . In this variant, the central shroud  31  of each ring sector  10  does not comprise an orifice  39 . 
     In the fourth embodiment, the ring sector  10  is fastened directly to the bearing shroud  356  using the screw  19  and the fastening part  20 . The bearing shroud  356  comprises an orifice  3560  traversed by the screw  19 . The orifice  3560  has a diameter smaller than that of the screw head  190 . 
     The radial securing of the ring sector  10  with the ring support structure  3  is carried out using the screw  19 , whose head  190  bears on the bearing shroud  356  of the annular flange  35 , and the fastening part  20  screwed to the screw  19  and fastened to the axial attachment tab  17 ′ of the ring sector  10 , the screw head  190  and the fastener  20  exerting forces of opposite directions to hold together the ring  1  and the ring support structure  3 . 
     In each of the embodiments of the invention illustrated in  FIGS. 1 to 6 , each ring sector  10  further comprises rectilinear bearing surfaces  110  mounted on the faces of the upstream and downstream radial attachment tabs  14  and  16  in contact respectively with the first tab  33  of the annular flange  35  and the second radial annular clamp  36 , that is to say on the upstream face  14   a  of the upstream radial attachment tab  14  and on the downstream face  16   b  of the downstream radial attachment tab  16 . In a variant, the rectilinear bearings could be mounted on the first tab  33  of the annular flange  35  and on the second downstream radial annular clamp  36 . 
     The rectilinear bearings  110  allow having controlled sealing areas. Indeed, the bearing surfaces  110  between the upstream radial attachment tab  14  and the first tab  33  of the annular flange  35  on the one hand, and between the downstream radial attachment tab  16  and the second radial annular clamp  36  on the other hand, are comprised in the same rectilinear plane. 
     More precisely, having bearings on radial planes allows overcoming the effects of de-cambering in the turbine ring  1 . 
     A method for producing a turbine ring assembly corresponding to that represented in  FIG. 1 , that is to say according to the first embodiment illustrated in  FIGS. 1 to 3 , is now described. 
     Each ring sector  10  described above is made of ceramic-matrix composite material (CMC) by formation of a fibrous preform having a shape close to that of the ring sector and densification of the ring sector by a ceramic matrix. 
     For the production of the fibrous preform, it is possible to use ceramic fiber yarns, for example SiC fiber yarns, such as those marketed by the Japanese company Nippon Carbon under the name “Hi-NicalonS”, or carbon fiber yarns. 
     The fibrous preform is advantageously made by three-dimensional weaving, or multilayer weaving with arrangement of debonding areas allowing the portions of preforms corresponding to the attachment tabs  14  and  16  of the sectors  10  to be spaced apart. 
     The weaving can be of the interlock type, as illustrated. Other weaves of three-dimensional or multilayer weaving can be used such as for example multi-plain or multi-satin weaves. Reference can be made to document WO 2006/136755. 
     After weaving, the blank can be shaped to obtain a ring sector preform which is consolidated and densified by a ceramic matrix, the densification can be achieved in particular by gas-phase chemical infiltration (CVI) which is well known per se. In a variant, the textile preform can be a little cured by CVI so that it is rigid enough to be manipulated, before raising liquid silicon by capillarity in the textile for carrying out the densification (“Melt Infiltration”). 
     A detailed example of manufacture of CMC ring sectors is in particular described in document US 2012/0027572. 
     The ring support structure  3  is for its part made of a metal material such as a Waspaloy® or inconel 718® or C263® alloy. 
     The production of the turbine ring assembly is continued by the mounting of the ring sectors  10  on the ring support structure  3 . 
     For this, the ring sectors  10  are assembled together on an annular tool of the “spider” type including, for example, suckers configured to each hold a ring sector  10 . 
     Then, the two second pins  120  are inserted into the two orifices  3650  provided in the third portion  365  of the second radial annular clamp  36  of the ring support structure  3 . 
     The ring  1  is then mounted on the ring support structure  3  by inserting each second pin  120  into each of the orifices  180  of the second lugs  18  of the downstream radial attachment clamps  16  of each ring sector  10  forming the ring  1 . 
     All the first pins  119  are then placed in the orifices  170  provided in the first lugs  17  of the radial attachment tab  14  of the ring  1 . 
     Then, the annular flange  35  is fastened to the ring support structure  3  and to the ring  1 . The annular flange  35  is cold-mounted on the ring support structure  3  in contact with the abutment  32 . During the temperature rise of the annular flange  35 , the shrink-fitting occurs at the two radial contacts. 
     In order to radially hold the ring  1  in position, the annular flange  35  is fastened to the ring by inserting each first pin  119  into each of the orifices  170  of the first lugs  17  of the upstream radial attachment tabs  14  of each ring sector  10  forming the ring  1 . 
     The ring  1  is thus axially held in position using the first tab  33  of the annular flange  35  and the second radial annular clamp  36  bearing respectively upstream and downstream on the rectilinear bearing surfaces  110  of the respectively upstream  14  and downstream  16  radial attachment tabs. During the installation of the annular flange  35 , an axial pre-stressing may be applied to the first tab  33  of the annular flange  35  and to the upstream radial attachment tab  14  to overcome the effect of differential expansion between the CMC material of the ring  1  and the metal of the ring support structure  3 . The first tab  33  of the annular flange  35  is maintained in axial stress by mechanical elements placed upstream as illustrated in dashed lines in  FIG. 3 . 
     The ring  1  is radially held in position using the first and second pins  119  and  120  cooperating with the first and second lugs  17  and  18  and the orifices  3540  and  3650  of the annular flange  35  and the radial annular clamp  36 . 
     The invention thus provides a turbine ring assembly allowing to maintain each ring sector in a deterministic manner while allowing, on the one hand, the ring sector, and by extension the ring, to deform under the effects of temperature rises and pressure variations, and in particular independently of the interface metal parts and, on the other hand, while improving the sealing between the off-flowpath sector and the flowpath sector and while simplifying manipulations and reducing their number for the mounting of the ring assembly. 
     In addition, the invention provides a turbine ring assembly comprising an upstream annular flange dedicated to take up the DHP force and thus to induce low levels of forces in the CMC ring, a contact abutment between the annular flange dedicated to take up the DHP force and the annular flange used to maintain the ring, the abutment allowing to ensure the non-contact of the low portions of the two flanges upon tilting of the upstream flange. The turbine ring assembly according to the invention also allows controlling the rigidity at the upstream and downstream axial contacts between the CMC ring and the metal casing. As a result, the sealing is ensured in all circumstances without inducing too high axial forces on the ring.