Abstract:
The retaining hooks ( 32 ) of a flange ( 22 ) of straightening sectors ( 20 ), bearing fixed blades in a turbomachine, are advantageously placed on an end plate ( 30 ) distinct from the casing ( 21 ) but assembled on the latter. In this way, these hooks no longer constitute weak points in the casing, as if they were all in one piece with it, assembly of the sectors ( 20 ) is facilitated, and the assembly is more rigid.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to assembling distributor sectors, bearing fixed blades, and ensuring correction of a gas flow in a casing enclosing stators of a turbomachine. 
   2. Description of the Related Art 
   The external flow of gases in turbomachines is delimited by distributor and ferrule sectors alternately mounted on the casing via overlapping hooks along the machine. With reference to  FIG. 1 , which illustrates an already known assembly, a straightening sector  1  bears a pair of assembly flanges  2  and  3 , respectively at the front and at the rear, and which are both fitted with end hooks  4  and  5 , extending in the longitudinal direction of the machine. The end hook  4  at the front is placed on a hook  6  of a casing  7  by a pair of mutual support faces  8 , which hold the sectors  1  against centripetal radial movements in the machine; the end hook  5  of the other flange  3  abuts radially towards the outside against the casing  7  by a pair of faces  9 , and towards the rear, against a second hook  10  of the casing  7 , by a pair of flat stop faces  11 . A clip  12  is placed around the second hook  10  below the second end hook  5  to also guard the sectors  1  against centripetal movements. The sectors  7  comprise more projections  13  towards the front, which encircle the rear of ferrule segments  14  between them and the first hooks. 
   This assembly holds the sectors  1  against the movements produced by the radial and axial forces oriented towards the rear during operation. Assembling these conventional devices is difficult to carry out, as the inclined sectors  1  have to be disposed in the casing  7  so that the rear flange  3  can be inserted in the second hook  10  before making them pivot about the end hook  4  at the front, so that the end hook  5  can be inserted behind the hooks  10  with a radial movement towards the outside. In order to effect this assembly, it is necessary to have two major sets of rotor-stator in the internal stream, with such a requirement being detrimental to the seals between rotor and stator. Finally, probably the most serious disadvantage of this assembly is the fact that the hooks  6  and  10  are integrally formed with the casing  7 , being subjected to significant mechanical and thermal stresses since they are near the stream of hot gases. As such, it becomes necessary to construct the hooks  6  and  10  and the casing  7  from a sufficiently resistant material even though the casing  7  is subjected to a lower temperature. In addition, due to the integral construction, fatigue cracking can appear in the hooks  6  and  10  and spread to the point where the entire casing  7  needs to be replaced. 
   Therefore, a need exists to rectify the design of the above-described casing, which is both costly and fragile, while enabling straightening sectors to be mounted more simply and producing a more rigid and less complex assembly. The instant invention satisfies these various requirements. 
   SUMMARY OF THE INVENTION 
   In its most general form, the present invention relates to the assembly of sectors of a distributor ring on a casing, the sectors comprising two flanges fitted with support faces on the casing, a hook being disposed beneath a curved edge of one of the flanges to support the sectors against centripetal movements, characterised in that the assembly comprises a support plate placed between the flanges; and means for fixing the support plate to the casing, the hook being placed on the support plate, the support plate and the casing comprising flat mutual support faces oriented in an axial direction, fixing means comprising slugs passing radially through the casing, the support plate being retained between the casing and the slugs, and the support plate having a mounting direction towards the flange having said curved edge. 
   The hook, which is the most stressed part of the assembly, may now be constructed with increased freedom for the choice of materials because it is disposed on a support plate and thus separated from the casing; it may be more easily replaced, if required; and finally, mounting is effected via linear movement of the sole support plate rather than the more complicated joint pivoting movement of the sectors in conventional assemblies. 
   The assembly may be further simplified if the hook comprises a conical external face at least on a central portion under the curved edge tapering towards the flange having the curved edge. 
   It should be noted that documents U.S. Pat. No. 3,892,497 and U.S. Pat. No. 5,618,161 disclose arrangements where distributor sectors are retained by carriers of a hook which is thus separated from the casing, but the end plates are placed otherwise and are not mounted by translation, but by tilting. They must be divided into sectors: the arrangement and mounting remain complicated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other arrangements, which can be adapted in addition to the above, multiply the advantages, which the invention provides. Its various aspects will emerge better from the following description of the figures, in which: 
       FIG. 1  already described, has the characteristic of an earlier assembly, 
       FIG. 2  shows the assembly of the invention, and 
       FIG. 3  evokes the mode of assembly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The casing is now designated by reference numeral  21 , but its general form resembles that of the previous one; its diameter decreases towards the front and its form is thus slightly conical at the point of the invention. 
   The new straightening sectors are designated by reference  20  and comprise, as before, flanges  22  and  23  at the front and at the rear are fitted with end hooks  24  and  25 , both curved towards the rear; the end hooks  24  and  25  are here supported on the casing  21  by mutual support faces  26  and  27 , oriented in a radial direction, and by flat support faces  28  and  29 , axially oriented and opposite to each other. Contrary to the above-noted conventional design, assembly of the new straightening sectors is simplified and made more stable. An essential element of the invention is a support plate  30  supported on the casing  21  by flat mutual support faces  31  and oriented in an axial direction, the support plate  30  comprising on its edge a hook  32  facing the front and passing under the end hook  24  of the first flange  22 . The support plate  30  extends on a full turn. As previously, the straightening sectors  20  bear towards the front a projection  33  for retaining the adjacent ferrule sectors  34  underneath. Another projection  35  can be noticed; it is found under the second end hook  25  and its purpose is to support ferrule sectors  35  opposite the preceding. 
   Slugs  37  are adjusted through the casing  21  in a radial direction, and keep the support plate  30  supported against the casing  21  by engaging behind the latter to block it axially. 
   The hook  6  of the conventional casing  7  has been eliminated, with the hook  32  of the support plate  30  replacing it, such that a weak point of the casing  7  has been eliminated. The hook  10  for the other flange  3  has not been eliminated completely, since the second end hook  25  is placed behind a groove  19  of the casing  21 , which ensures the same support functions for the flange  23 , but it should be emphasised that the groove  19 , more massive and less protruding than the hook  10 , is much less stressed. Such modification of the hook  10  in the design of  FIG. 1  would have been of little use, as long as the hook  6  was there, which had to project substantially to enable the straightening sectors  1  to be mounted by an axial then pivoting movement. In summary, eliminating the weak point of the casing  7  constituted by the hook  6  allows the elimination of the other weak point constituted by the hook  10 . 
   Eliminating hooks  6  and  10  or replacing them by simpler structures decreases stress and the risk of failure of the casing  21 , while, at the same time, simplifying the casing manufacture process. The casing  21  now may be constructed from a less resistant material than the one used for casing  7 . Only the hook  32  and its support plate  30  remain subjected to high temperatures, thus requiring the use of a more resistant material. 
   As shown in  FIG. 3 , the straightening sectors  20  may be raised by a purely radial movement as the pairs of support faces  28  and  29  slide upward as the support plate  30  is moved towards the front as far as the stop of faces  31 . The hook  32  is fitted with an external face of which at least one central surface  38  is conical, tapering towards the front, making the first end hook  24  rise onto this surface  38 , while the hook  32  advances. When the end hook  24  passes this surface  38  to rest on a rear cylindrical portion  39  of the upper surface of the hook  32 , the straightening sectors  20  have been put into place. Then, the slugs  37  are to be introduced to their bores to axially block the support plate  30 . 
   The movement of the support plate  30  towards the front can be made very simply by a special tool, as often found for carrying out assembly and disassembly work of turbomachines. An appropriate tool would comprise a platen mounted in the casing supporting the straightening sectors  1 , as well as thrusters for pressing on the rear of the support plate  30  through holes  42  hollowed through the other flange  23 . 
   It should be emphasised that one of the main stresses being exerted on the straightening sectors  1  or  20  is a force to the rear, produced by the flow of air across the latter. This force is translated by a tilting motion around support points of the sectors on the casing  7  or  21  and by a centripetal radial force on the hook  6  or  32 , which makes it vulnerable, but the consequences of breaking would be less serious on the hook  32  than on the hook  6 , since it would be enough to replace the support plate  30  instead of the entire casing  7 . The hook  32  offers radial support to the inside on sectors  20 , just like the hook  6  on the sectors  1 , since the end hook  24  is placed on the latter, but it offers more rigid assembly than the hook  6 , even though it is not all in one with the casing  21 , by keeping the end hook  24  pressed against the casing  21  on the support faces  26 . The diameter of a surface portion of the rear portion  39  can be selected to exert the desired pressing force at this point. 
   The conicity of the surface  38  always allows the support plate  30  to be placed without difficulty, with the hook  32  acting as a corner. Assembly is still stiff, while being simplified, if the ferrule sectors  36  to the rear comprise a projecting front end  43  placed on the projection  35  and introduced on the end hook  25  of the flange  23  in the place of the clip  12 . 
   If the support plate  30  must be replaced, it is easy to extract it if tapping points are put in place, such as internal screw threads  53 , which allow it to be gripped by threaded axes introduced through the holes  42 , and to be extracted by drawing it to the rear until the straightening sectors  20  are released. 
   It should be noticed that the end hook  24  of the first flange  22  was directed to the rear, and the hook  32 , with which the first flange is assembled, was directed towards the front, contrary to the traditional arrangement as in  FIG. 1 . The advantage of this inversion, made possible by eliminating the pivoting movement when mounting the straightening sectors  20 , is that the hook  32  and its support plate  30  are lodged between the flanges  22  and  23 , in a volume isolated from the temperature of the gases by the distributor sector  20 .