Abstract:
A positioning member for presenting a ring segment for a turbine wheel mounted to rotate about an axis inside a casing. The member presents: a fastener portion for fastening to the casing; a resilient portion forming a spring; and a bearing portion connected to the resilient portion and serving to bear axially against the ring segment such that when the member is mounted the ring segment is pressed axially against a portion of the casing by the positioning member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field Of The Invention 
     The present invention relates to the field of turbomachine turbines, and more particularly for those that are to be found in gas turbines. 
     The present invention relates more precisely to a turbomachine having a positioning member for positioning a ring segment for a turbine wheel that is mounted to rotate about an axis inside a casing. 
     2. Description Of The Related Art 
     Traditionally, the wheels of turbomachine turbines, such as the high pressure turbines of helicopter gas turbines, are surrounded by a ring that is concentric about the turbine wheel. As a component element of the stator of the turbomachine, the ring forms an outer shroud for the turbine stage. In other words, the burnt gas leaving the combustion chamber flows between the hub of the turbine wheel and the ring in such a manner as to drive the turbine wheel in rotation. 
     Generally, the ring is constituted by a plurality of ring segments, also referred to as ring sectors, that are placed contiguously one after another. 
     These ring segments are generally not fastened securely to the turbomachine casing since they are liable to expand axially and radially as a result of the high temperatures conveyed by the burnt gas. 
     The ring segments are held inside the casing by one or more positioning members. 
     Furthermore, it is common practice to provide for cooling the ring segments by causing a cooling fluid to flow around the ring, the cooling fluid possibly being bled from the compressed air that is generated by the turbomachine compressor. 
     Nevertheless, such a configuration presents the drawback of creating leakage zones, and it can readily be understood that the burnt gas might penetrate into the cooling circuit or, conversely, that the cooling fluid might penetrate into the gas flow section located upstream or downstream from the turbine wheel, which would penalize the efficiency of the turbomachine, with the penalty being worse when ejection takes place downstream. 
     To avoid leaks, document U.S. Pat. No. 5,988,975 proposes a solution that consists in providing radial sealing by means of a ring that keeps the ring segments pressed radially against the casing. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a turbomachine including an alternative positioning member that serves to provide sealing between the gas flow section and the cooling circuit. 
     The invention achieves this object by the fact that the positioning member presents:
         a fastener portion for fastening to the casing;   a resilient portion forming a spring; and   a bearing portion connected to the resilient portion and serving to bear axially against the ring segment in such a manner that when said member is mounted, the ring segment is pressed axially against a portion of the casing by the positioning member.       

     In the meaning of the present invention, the terms “axial” and “radial” should be considered relative to the axis and to a radius of the turbine wheel, whereas the term “downstream” should be understood with reference to the flow direction of burnt gas through the turbine. 
     Thus, once each ring segment is held axially against an (upstream or downstream) portion of the casing that is normal to the axis of rotation, by means of the axial force exerted by the bearing portion of the positioning member, it follows that sealing is provided between the cooling fluid circuit and the gas flow section. 
     In other words, with the positioning member fastened to the casing by means of the fastener portion, the axial force generated by the resilient portion is transmitted to the bearing portion and then to the ring segment. In the absence of the positioning member, the ring segment is suitable for moving a little relative to the casing in an axial direction, so it can readily be understood that the bearing portion pushes the ring segment against the casing portion, thereby pressing it against said casing portion. Preferably, the bearing portion pushes the ring segment against the portion of the casing that is downstream. 
     In another variant, the fastener portion also bears against the ring segment. 
     Preferably, the resilient portion is located between the fastener portion and the bearing portion. 
     Preferably, the casing portion against which the ring segment is pressed is normal to the axis of rotation of the turbine wheel. 
     Advantageously, the positioning member includes at least a first arm carrying the fastener portion. 
     Preferably, this first arm is designed to extend axially. 
     In a first variant, the fastener portion is in the form of a tab extending orthogonally relative to a plane in which the first arm extends. 
     Thus, when the positioning member is mounted, the tab extends radially. It serves in particular to be clamped between two component portions of the casing. 
     In another variant, the fastener portion comprises a hole with an axis that extends orthogonally relative to a plane in which the first arm extends. In this variant, the casing is fastened, by way of example and not necessarily, by means of a nut-and-bolt system, such that the first arm of the positioning member is prevented from moving axially relative to the casing. 
     Advantageously, the resilient portion is constituted by a corrugated tongue, this tongue being a fine metal plate, for example, e.g. a flexible sheet, presenting a plurality of successive folds. Nevertheless, it is possible to devise other types of resilient portion. 
     Furthermore, the corrugations are preferably located between the fastener portion and the bearing portion. 
     In a first embodiment, the positioning member is in the form of a tongue having a first end constituting the fastener portion that presents an orthogonal tab and a second end constituting the bearing portion that is curved, with the middle portion located between the first and second ends being corrugated so as to constitute the resilient portion. 
     In another embodiment, the positioning member comprises at least a first arm carrying the fastener portion, together with at least a second arm including the resilient portion, the resilient portion preferably being constituted by corrugations. 
     In an advantageous aspect of the invention, the first and second arms are parallel and extend from a plate, and the fastener portion is situated at one end of the first arm remote from the plate, while the bearing portion is situated at the end of the second arm that is remote from the plate. 
     An advantage of the fastener portion being close to the bearing portion (in particular the point where the positioning member is fastened to the casing being close to the bearing portion) is to limit the effect of the deformation (under temperature effect) of the casing, of the segments, or of the positioning system between the bearing portion and the fastener portion. This also limits the forces that are applied to the segments and consequently improves the behavior of the ring segments. 
     Preferably, the positioning member further includes a third arm similar to the second arm extending from the plate, the first arm extending between the second and third arms. Thus, the positioning member is E-shaped, with the three branches thereof being constituted by the first, second, and third branches. More precisely, these three branches extend axially when the positioning member is mounted with the ring segment. 
     An advantage of this positioning member is that it also makes it possible to retain two consecutive ring segments in an azimuth direction. To do this, each of the ring segments is held in an azimuth direction between the first arm of a first positioning member situated at one of the ends of the segment and the first arm of a second positioning member situated at the other end of the segment. 
     Furthermore, the first arm also preferably includes a second resilient portion located between the plate and the fastener portion, thereby further improving the resilience of the positioning member. 
     In another embodiment of the invention, the first arm forms a plate for covering a junction with a gap defined between two contiguous ring segments. Under such circumstances, one of the ends of the covering plate preferably includes the above-mentioned tab for fastening the positioning to the casing. Thus, the covering plate serves to improve sealing by covering the gap that might exist between the two ends of two contiguous segments. 
     In another embodiment, the resilient portion is constituted by a pair of arms in a V-shape extending from a plate. Under such circumstances, the resilient portion is constituted by a pair of resilient arms, which pair is suitable for returning to its initial shape after being deformed. 
     In this embodiment, the bearing portion is advantageously constituted by the ends of said arms, which ends are designed to bear axially against the ring segment so as to press it against the (downstream or upstream) portion of the casing that extends normally to the axis of rotation. 
     The present invention thus provides a turbomachine, e.g. but not necessarily for a helicopter, the turbomachine including a casing, a turbine wheel mounted to rotate about an axis inside casing, and a ring arranged concentrically around the turbine wheel, said ring being made up of at least first and second ring segments, said turbomachine further including at least one positioning member as described above, the positioning member being fastened to the casing and exerting axial thrust on at least one of ring segments so as to press it axially against a portion of the casing. 
     In a preferred manner, the positioning member presents a fastener portion for fastening it to the casing, which fastener portion is situated close to said casing portion in order to avoid being affected by the axial expansion of the ring segments. 
     Said portion is preferably a downstream portion of the casing. 
     Advantageously, the positioning member includes a first arm fastened to the casing, a resilient second arm exerting axial thrust on the first ring segment, and a resilient third arm exerting axial thrust on the second ring segment, so as to press the ring segments axially against a portion of the casing. Preferably, the first and second ring segments are contiguous. 
     In preferred manner, the turbomachine of the invention further includes at least one plate covering the junction between the first and second contiguous ring segments, the covering plate being held radially against the segments by the positioning member. 
     Preferably, the positioning member includes a resilient portion constituted by a tongue that presents corrugations, the tongue bearing radially against the cover plate in order to hold it radially against the contiguous ends of the first and second ring segments. 
     Preferably, the corrugations present radial flexibility so as to absorb vibration of the cover plate. Furthermore, this radial flexibility serves to hold the cover plate radially against the ends of the segments whatever the pressure difference that might exist between the gas flow section and the cavities inside the casing. 
     Another advantage of the positioning member is that it serves to absorb vibration of the segments. 
     By virtue of its corrugated portions, the positioning member may also bear radially both against the ring sector or the sealing tongue, and against the portion of the casing or ring support. Contact against these two parts serves to reduce or even eliminate any radial offset or clearance between the ring sectors, thereby ensuring controlled radial positioning of these sectors: this is particularly advantageous for ensuring control over clearances in the turbine, and thus over the performance thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention can be better understood and its advantages appear better on reading the following description of embodiments given as non-limiting examples. The description refers to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a first embodiment of the positioning member of the turbomachine of the invention; 
         FIG. 2  is a fragmentary perspective view of a turbine ring, showing two ring segments held together by a positioning member of  FIG. 1 ; 
         FIG. 3  is a fragmentary axial section view of a turbomachine showing the positioning member of  FIG. 1  when mounted; 
         FIG. 4  is a perspective view of a second embodiment of the positioning member of the invention; 
         FIG. 5  is a fragmentary axial section view of a turbomachine having the  FIG. 4  positioning member mounted therein; 
         FIG. 6  is a perspective view of a third embodiment of the positioning member of the turbomachine of the invention; 
         FIG. 7  is a fragmentary axial section view of a turbomachine having the  FIG. 6  positioning member mounted therein; 
         FIG. 8  is a perspective view of a fourth embodiment of the positioning member of the turbomachine of the invention; and 
         FIG. 9  is a fragmentary axial section view of a turbomachine having the  FIG. 8  positioning member mounted therein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIGS. 1 to 3 , there follows a description of a first embodiment of the invention. 
     In the invention, a positioning member  10  serves to position a turbine ring segment relative to a casing of the turbomachine in which the turbine is rotatably mounted. 
     As can be seen in  FIG. 1 , the positioning member  10  is generally in the form of the letter “E”. It comprises a plate  12  from which there extend orthogonally a central first arm  14  together with second and third arms given references  16  and  18 , said arms being such as to constitute the three branches of the letter “E”. Said arms are substantially parallel to one another. 
     Furthermore, the positioning member  10  presents thickness that is small compared with its other dimensions. More precisely, each of the arms is in the form of a corrugated tongue. 
     In accordance with the invention, the positioning member  10  presents a fastener portion  20  that, in this embodiment, is constituted by a hole  22  formed in the free end  14   a  of the first arm  14 , i.e. in its end remote from the plate  12 . In similar manner, the free ends of the second and third arms  16  and  18  are referenced  16   a  and  18   a.    
     This fastener portion  20  is for fastening to the casing  62  of the turbomachine so as to fasten the positioning member  10  to the casing of the turbomachine. This aspect is described below. 
     Each of the first, second, and third arms  14 ,  16 , and  18  also includes a resilient portion  24 ,  26 , or  28  that forms a spring. 
     Each of these resilient portions  24 ,  26 , and  28  is in the form of corrugations. In other words, each of the arms  14 ,  16 , and  18  presents a succession of folds so as to form corrugations, it being specified that the folding axes extend transversely relative to said arms. 
     These corrugations  24 ,  26 , and  28  can be deformed in the longitudinal direction of the arms  14 ,  16 , and  18 , but they tend to return to their original shape. The corrugations  26  and  28  of the second and third arms constitute compression springs in the sense that if an attempt is made to move the free ends towards the plate, then the corrugations as deformed in this way generate an opposing force referred to as a compression force. In contrast the corrugation  24  constitutes a traction spring. An advantage of this provision is explained below. 
     Furthermore, the positioning member  10  has two bearing portions  30  and  32  that are constituted by the free ends  16   a  and  18   a  of the second and third arms  16  and  18 . As can be seen in  FIG. 1 , the free ends  16   a  and  18   a  are curved so as to form hook-like shapes. 
     Furthermore, and preferably, the second and third arms  16  and  18  are slightly longer than the first arm  14 . 
     With reference to  FIGS. 2 and 3 , there follows an explanation of how the positioning member  10  is mounted. 
     Conventionally, the ring of a turbine is made up of a plurality of ring segments that are placed contiguously end to end.  FIG. 2  shows the first and second ring segments  50  and  50 ′ placed in abutment via their respective ends  50   a  and  50 ′ a.    
     The shape of the ring is such that it defines an axial direction, a radial direction, and an azimuth direction, these three directions being mutually orthogonal. As mentioned above, the ring is designed to be placed around the turbine wheel of the turbomachine. 
     A first advantage of the positioning member  10  is that it holds the ends  50   a  and  50 ′ a  so as to prevent the ring segments  50  and  50 ′ from turning about the axis of the turbine wheel. To do this, each of the ring segments  50  and  50 ′ carries a spline  52  or  52 ′ extending in a transverse direction relative to the ring segment while also projecting radially, such that when the positioning member  10  is mounted, the plate  12  extends along the azimuth direction of the ring segments  50  and  50 ′, and while the first and second arms  14  and  16  lie on either side of the spline  52 ′ of one of the ring segments  50 ′, while the first and third arms  14  and  18  of the member  10  lie on either side of the spline  52  of the other ring segment  50 . Consequently, it can be understood that the positioning member  10  prevents the ends  50   a  and  50 ′ a  of each of the ring segments  50  and  50 ′ from moving in an azimuth direction. Preferably, each segment  50  and  50 ′ is held at each of its azimuth ends by two respective members  10 . 
     As can be seen in  FIG. 2 , the bearing portions  30  and  32  of the positioning member  10 , i.e., in this example, the free ends  16   a  and  18   a  of the second and third arms  16  and  18 , come to bear axially against respective peripheral edges  51  and  51 ′ of the first and second ring segments  50  and  50 ′. An advantage of this configuration is explained below. 
     Another advantage of the present invention can be better understood with reference to  FIG. 3  which shows details of how the ring segment  50  and  50 ′ and the positioning member are mounted in the casing  62  of the turbomachine  60 . 
     As mentioned above, the ring segments  50  and  50 ′ are placed around the wheel of the high pressure turbine so as to cover the blades  64  of said turbine. Arrows F represent the flow direction of burnt gas leaving the combustion chamber that is located upstream from the high pressure turbine. 
     As can be understood from  FIG. 3 , the axial ends  50   b  and  50   c  of the ring segments  50  present axial swellings  66  and  68  for co-operating with projections carried by the upstream and downstream portions  62   a  and  62   b  of the casing  62  so that the ring segment  50  is held radially in the casing  62 . Nevertheless, the ring segments  50  are mounted in the casing  62  with a small amount of axial clearance. 
     Furthermore, it can be seen that casing  62  and the ring segments  50  and  50 ′ are arranged in such a manner as to define an annular passage P to allow cooling fluid to flow around the ring. Orifices  70  are formed in the casing  62  to enable the cooling fluid to be brought into contact with the ring segments in order to cool them. 
     In accordance with the invention, the positioning member  10  is fastened to the casing  62  by means of a fastener portion  20  by using a locking member of the nut and bolt or peg type  72 , for example. 
     When the positioning member  10  is in the mounted position, its arms  14 ,  16 , and  18  extend in the axial direction of the turbine wheel, while the plate  12  is housed between the upstream swelling  66  of the ring segment and the casing  62 . 
     Furthermore, the point where the positioning member  10  is fastened, i.e. in this example the nut  72 , is located relative to the downstream projection  62   b  of the casing  62  in such a manner that the corrugations  24 ,  26 , and  28  of the member  10  are subjected to continuous axial compression in their longitudinal direction, thus causing the free ends  16   a  and  18   a  of the second and third arms  16  and  18  to exert axial thrust that is directed downstream against the peripheral edges  51 ,  51 ′ of the ring segments  50  and  50 ′. As a result, the ring segments  50  and  50 ′ are pressed axially against a downstream portion of the casing, which portion is constituted in this example by the above-mentioned downstream projection  62   b.  Contact between the ring segments  50  and  50 ′ and the downstream portion of the casing  62   b,  which contact takes place via a contact zone given reference C, serves to provide sealing between the annular passage P and the downstream flow section V, thereby preventing the cooling fluid that flows in the annular passage from flowing into the downstream flow section V. 
     Another advantage is that the cooling fluid flowing in the annular passage also cools the corrugations  30  and  32  that might be caused to heat up. This makes it possible to control the lifetime of the corrugations, by optimizing their operating temperature. 
     Finally, in  FIG. 3 , it can be seen that the positioning member  10 , and more precisely the corrugations  24  of the first arm  14 , serves to keep a plate  54  radially against the contiguous ends of the first and second ring segments  50  and  50 ′. This plate  54  covers the junction  56  defined between the ends  50   a  and  50 ′ a  of the two ring segments as to improve sealing between the annular passage and the flow section in which the blades  64  of the turbine wheel move. 
     Another advantage of the positioning member  10  is that it pushes the ring segment at each of its axial ends in an azimuth direction, thereby improving contact between the ring segment  50  and the downstream portion  62   b  of the casing  62 , and thus improving sealing. The positioning of the bearing at the ends of the segments  50  and  50 ′ also serves to reduce any risk of the segments jamming in the grooves of the casing  62 . 
     With reference to  FIGS. 4 and 5 , there follows a description of a second embodiment of a positioning member  110  in accordance with the present invention. In this embodiment, elements that are identical to those of the first embodiment are given the same numerical references plus one hundred. 
     The positioning member  110  is in the form of a single arm  114  constituted by a tongue. Like the first embodiment, the arm or tongue  114  presents a resilient portion  124  forming a compression spring, this resilient portion being constituted by corrugations, the corrugations being made by folding the tongue  114  several times. 
     In addition, the member  110  presents a fastener portion  120  constituted by a tab that extends orthogonally relative to a plane in which the first arm  114  extends. This tab  120  may be made by folding the tongue  114  for example. 
     In order to be consistent with the first embodiment, it may be stated that the member  110  presents a plate  112  located between the tab  120  and the corrugations  124 . 
     Finally, the tongue  114  has a bearing portion  130  constituted by the end of the tongue that is remote from the tab  120 , which end is curved. 
       FIG. 5  shows the positioning member  110  mounted in the turbomachine  60 . 
     The positioning member  110  is fastened to the casing  110  by the tab  120 . To do this, the tab is received radially between the ring segment  50  and the casing  62 . 
     The tongue  114  extends axially so that its free end  130  comes to bear against the edge  51  of the ring segment  50 . 
     Furthermore, the length of the tongue  114  is selected so that when the ring segment is positioned axially against the downstream portion  62   b  of the casing, the corrugations  124  of the member  110  are compressed axially. It follows that the end  130  of the tongue exerts axial thrust against the peripheral edge  51  of the ring segment  50 , thereby causing the ring segment to be pressed axially against the downstream portion  62   b  of the casing  62 , and specifically to be pressed against the contact zone C. 
     It can thus be understood that the positioning member  110  advantageously keeps the ring segment  50  in axial contact with the downstream portion  62   b  of the casing  62 . 
     Furthermore, the corrugations  124  of the member  110  enable the plate  54  to be held radially against the ends of the ring segments. 
     With reference to  FIGS. 6 and 7 , there follows a description of a third embodiment of the invention. 
     In this embodiment, elements identical to those of the first embodiment are given the same numerical references plus two hundred. 
     The positioning member  210  in the third embodiment is generally E-shaped. In this respect, it has a central first arm  214  that forms a plate that is straight (without corrugations) for covering the junction between the two ends of the ring segments. In other words, the positioning member advantageously makes it possible to omit a junction-covering plate as described above, since the first arm  214  specifically performs the role of said junction-covering plate. 
     At one of the ends of the first arm  214  there is a plate  212  carrying a fastener portion  220  that is constituted by a tab similar to that of the second embodiment. 
     The plate  212  has second and third arms  216  and  218  projecting therefrom, similar to the second and third arms of the first embodiment. I.e. these arms include respective first and second resilient portions  226  and  228  forming corrugations. 
     Furthermore, the free ends  216   a  and  218   a  constitute bearing portions  230  and  232  similar to those of the first embodiment. 
       FIG. 7  shows the positioning member  200  when mounted in the casing  60 . The tab  220  bears axially against the upstream portion  62   a  of the casing  62 , while the free ends  216   a  and  218   a  of the second and third arms come to bear against the peripheral edge  51  of the ring segment  50  so as to press it axially against the downstream edge  62   b  of the casing  62 . To do this, the second and third arms  216  and  218  are slightly longer than the distance between the upstream portion  62   a  of the casing and the edge  51  such that the corrugations  226  and  228  are in compression. As in the first embodiment, this compression of the corrugations generates axial thrust on the edges  51  of the contiguous ring segments downstream from the free ends  216   a  and  218   a  so that the ring segments  50  and  50 ′ are pressed axially against the downstream portion  62   b  of the casing  62 . 
     With reference to  FIGS. 8 and 9 , there follows a description of a fourth embodiment of the invention. The positioning member  310  has a plate  312  provided with a hole  314  constituting a fastener portion for fastening to the casing  62 . 
     A pair of arms  318 ,  320  constituting a V-shape extends in a curved portion  316  of the plate  312 . The positioning member  310  is made of a material that presents a certain amount of stiffness, such that each of the arms  318  and  320  constitutes a resilient portion forming a compression spring. It can be understood that if any attempt is made to spread the arms apart from each other, they exert a return force that tends to bring the arms back towards their original, rest position. In other words, the arms  318  and  320  act like spring blades. 
     When the positioning member  310  is mounted in the manner shown in  FIG. 9 , it is fastened to the casing  62  by a nut  322  similar to that of the first embodiment. 
     Each arm is provided at its end  318   a,    320   a  with a bearing portion that is hook-shaped. 
     In this embodiment, the ends  318   a  and  320   a  of the pairs of arms  318  and  320  serve to bear against the peripheral edge  51  in such a manner as to press the ring segments  50  and  50 ′ against the downstream portion  62   b  of the casing  62 . In this position, the arms  318  and  320  are slightly deformed so as to maintain axial pressure against the ring segment  50 . The position of the fastener point, i.e. the nut or peg  322 , or the length of the arms  318 ,  320  should be selected so as to obtain this effect.