Abstract:
A venting pipe used to guide a gas stream in a turbojet including at least one hollow rotary shaft inside which the pipe is mounted, the pipe extending generally along an axis. The pipe includes at least two pipe segments configured to be aligned longitudinally and assembled to one another while retaining a degree of freedom in relative translation thereof, at least one of the segments including a deformable mechanism configured to deform radially as the pipe segments are tightened against one another in the shaft of the turbojet, to bear on the shaft. The pipe structure facilitates mounting of the pipe.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a turbojet venting pipe, to a method for mounting such a pipe and to a turbojet provided with one such pipe. 
     2. Description of the Related Art 
     A turbine engine for an aircraft generally comprises, from upstream to downstream in the flow direction of the gases, a fan, one or more compressor stages, for example a low-pressure compressor and a high-pressure compressor, a combustion chamber, one or more turbine stages, for example a high-pressure turbine and a low-pressure turbine, and a gas exhaust nozzle. One turbine may correspond to each compressor, both being connected by a shaft, thus forming for example a high-pressure core and a low-pressure core. 
     A turbojet generally has, substantially at the upstream end of the high-pressure core, an “upstream compartment” containing components of the rolling bearing and gearing type. It furthermore has, substantially at the downstream end of the high-pressure core, a “downstream compartment” containing components of the rolling bearing and gearing type. These compartments are immersed in an atmosphere containing oil for lubrication of the various components. A gas flow furthermore passes through them, in particular for ventilation purposes. In order to prevent the oil from being transported out of the compartments by the gas flow, the gases are evacuated in “deoilers”, which are generally formed by radial passages formed in the low-pressure shaft and on the wall of which the oil is captured in order to be reinjected into the corresponding compartment, by centrifugal effect. The deoilers communicate with a (likewise rotating) pipe referred to as a venting pipe, in the interior of which the gases are transported from the deoilers in order to be ejected at the exit of the venting pipe, generally at the nozzle of the turbojet. 
     The venting pipe extends inside the low-pressure shaft, concentrically therewith, the low-pressure shaft for its part extending inside the high-pressure shaft, concentrically therewith. The venting pipe rotates with the low-pressure shaft; it generally extends over a majority of the longitudinal dimension of this shaft. The venting pipe makes it possible to guide the gases and, in particular, to avoid contact of the oil-laden gases with the low-pressure shaft which, owing to the high temperature of the latter, could lead to coking phenomena of the oil in suspension in the gases. 
     In most known turbojets, the low-pressure shaft has a wall of variable thickness, the internal surface of its wall having a variable diameter along the shaft. The person skilled in the art conventionally refers to a so-called “bottle”-shaped shaft owing to the shape of its internal wall; the internal surface of the wall of such a shaft has a larger diameter in its central region than in its end portions. 
     The venting pipe generally has a wall of relatively small thickness compared with the thickness of the wall of the low-pressure shaft. Because of its slenderness, it needs a certain number of supports on the internal surface of the wall of the low-pressure shaft, not only at its ends but also in the central part. A mounting problem then arises, since the venting pipe needs to be mounted via an end of the shaft, which has a diameter less than the diameter of its central part but on the internal surface of which the pipe nevertheless needs to bear in order to ensure that it is held. This problem is solved in the prior art by using systems of conical rings and nuts which are mounted in the shaft before the pipe is mounted. These systems are complex and require sufficient clearance between the external surface of the wall of the venting pipe and the internal surface of the wall of the low-pressure shaft. 
     In certain recent turbojets, the diameter of the high-pressure shaft is reduced relative to that of previous turbojets. The size of the engine therefore requires that a wall of constant thickness be provided for the low-pressure shaft, with external and internal surfaces of constant diameters along the majority of the shaft, these diameters furthermore being reduced relative to those of the shafts of the prior art. The venting pipe must for its part have a diameter substantially equal to that of the venting pipes of the prior art, in order to ensure the discharge of an equivalent gas flow rate. For this reason, the space between the external surface of the venting pipe and the internal surface of the low-pressure shaft is small and makes it difficult to mount points of support. Notwithstanding, the presence of such points of support along the venting pipe remains necessary in view of its slenderness (about 2 meters in length with a diameter of 60 millimeters). 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a venting pipe which is easier to mount. The invention is derived from a problem in the case of turbojets with little clearance between the venting pipe and the low-pressure shaft; nevertheless, the Applicant does not intend to limit the scope of its rights to this application alone, the invention being more generally applicable and capable of offering its advantages in any type of turbojet. 
     This is why the invention relates to a pipe referred to as a venting pipe for guiding a gas flow in a turbojet comprising at least one hollow rotary shaft inside which said pipe is intended to be mounted, the pipe extending overall along an axis, which pipe is characterized in that it comprises at least two pipe segments arranged in order to be assembled with one another in longitudinal alignment while retaining a degree of freedom in their relative translation, at least one segment being provided with deformable means arranged in order to deform radially when the pipe segments are tightened against one another in the shaft of the turbojet, in order to bear on the shaft. 
     By virtue of the invention, mounting of the pipe in the shaft is facilitated since it is under the effect of the mounting of the two pipe segments in the shaft that the deformable means deform radially in order to bear on the shaft. 
     In particular, the deformable means are deformed radially under the effect of a relative longitudinal displacement of the two segments with respect to one another. 
     Owing to their radial deformation, the diameter of the deformable means is greater after mounting than before mounting; the sleeving of the pipe segments in the shaft thus takes place with a minimal diameter of the deformable means (and is therefore facilitated), while the completion of the mounting involves the deformable means deforming radially in order to bear on the shaft so as to hold the pipe therein. A venting pipe can thus be installed easily in a reduced volume inside the shaft, without intermediate supports, while optimizing its cross section for good circulation of the gas flow and good distribution of the pressures. 
     Preferably, the deformable means are arranged at one end of said segment. 
     According to one embodiment, the deformable means have, before mounting, a diameter less than the internal diameter of the shaft. The mounting is therefore facilitated since it can take place without contact between the deformable means and the shaft. 
     According to one embodiment, the deformable means comprise a ring formed from deformable material, for example deformable metallic material. 
     According to one embodiment in this case, the ring is formed from an elastomer; such a material is highly suitable for use in a turbojet. 
     According to another embodiment in this case, the ring is in the form of a deformable convex annular metal plate, for example formed from nickel alloy such as Inconel X750 (registered trademark). 
     According to one embodiment, one end of a first segment comprises a radial bearing edge for the ring and a second segment comprises an end portion arranged in order to bear on the ring in order to compress it longitudinally against the radial edge and thus deform it radially. Such a device is easy to manufacture and install. 
     According to one embodiment, the segments comprise means for blocking in rotation with respect to each other (or one another) in the assembled position. It is thus simple to secure the assembly to the rotary shaft. 
     According to one embodiment, the pipe comprises more than two pipe segments. 
     According to one embodiment, the pipe segments are arranged in order to be assembled with one another jointly with their mounting in the shaft, the deformable means being arranged in order to be deformed during this assembly. 
     According to one embodiment, the pipe segments are arranged in order to be assembled prior to their mounting in the shaft; preassembled in this way, the pipe segments form a unitary assembly of two or more elements connected together. In this way, it is easy to handle the preassembled pipe in one unit for mounting it in the shaft, this mounting therefore being less complex, more rapid and less demanding, particularly in terms of tooling cost. Furthermore, maintenance of the assembly is facilitated since it is easy to withdraw the entire pipe from the shaft. 
     In this case, the segments are assembled with one another then mounted in the shaft, the deformation of the deformable means in order to come in contact with the shaft taking place during this mounting in the shaft. 
     The invention also relates to a turbojet comprising at least one hollow rotary shaft inside which a pipe referred to as a venting pipe for guiding a gas flow is intended to be mounted, the pipe extending overall along an axis, which pipe is characterized in that it comprises the characteristics of the pipe presented above. 
     The invention relates to a method for mounting a pipe referred to as a venting pipe, for guiding a gas flow inside a hollow rotary shaft of a turbojet, the pipe being intended to extend overall along an axis and comprising at least two pipe segments arranged in order to be assembled with one another in longitudinal alignment while retaining a degree of freedom in their relative translation, at least one segment being provided with deformable means, which method is characterized in that it comprises the following steps:
         the pipe is mounted inside the shaft and   the deformable means are radially deformed by tightening said segments against one another, until they bear on the shaft of the turbojet in order to hold the pipe therein.       

     This method has the same advantages as the pipe described above. 
     According to one embodiment, the pipe segments are assembled with one another jointly with their mounting in the shaft, and the deformable means are deformed during this assembly. 
     According to another embodiment, the pipe segments are assembled with one another prior to their mounting in the shaft. In this case, the segments are assembled with one another then the pipe with its two segments is mounted in the shaft, the deformation of the deformable means in order to come in contact with the shaft taking place during this mounting. 
     The method may advantageously be carried out with the pipe presented above. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will be understood more clearly with the aid of the following description of the preferred embodiment of the venting pipe, the turbojet and the mounting method which correspond to the preferred embodiments of the invention, with reference to the appended plates of drawings, in which: 
         FIG. 11  represents an overall view in section of the turbojet according to a first embodiment of the invention; 
         FIG. 1  represents a schematic view in section of the low-pressure shaft and of the venting pipe of the turbojet of  FIG. 11 ; 
         FIG. 2  represents a detailed view of the downstream portion of the low-pressure shaft of  FIG. 1  during a first step of the method for mounting its venting pipe; 
         FIG. 3  represents a view in section of the abutment region of the segments of the venting pipe of the low-pressure shaft of  FIG. 1  during a second step of its mounting method; 
         FIG. 4  represents a view in section of the abutment region of the segments of the venting pipe of the low-pressure shaft of  FIG. 1  during a third step of its mounting method; 
         FIG. 5  represents a view in section of the abutment region of the segments of the venting pipe of the low-pressure shaft of  FIG. 1  during a fourth step of its mounting method; 
         FIG. 6  represents a view in section of the upstream region of the low-pressure shaft of  FIG. 1 ; 
         FIG. 7  represents a view in section of the abutment region of the segments of a venting pipe according to a second preferred embodiment of the invention; 
         FIG. 8  represents a view in section of the abutment region of the segments of a venting pipe according to a third preferred embodiment of the invention; 
         FIG. 9  represents a view in section of the abutment region of the segments of a venting pipe according to a fourth preferred embodiment of the invention; 
         FIG. 10  represents a view in section on the one hand of the abutment region and on the other hand of the upstream portion of the segments of a venting pipe according to a fifth preferred embodiment of the invention; 
         FIG. 12  represents a schematic view in section of a venting pipe according to a sixth embodiment of the invention; 
         FIG. 13  is a view in section of the abutment region of the upstream and intermediate segments of the venting pipe of  FIG. 12 , the lower part of  FIG. 13  showing this abutment region before compression of the deformable means and the upper part of  FIG. 13  showing this abutment region after compression of the deformable means; 
         FIG. 14  represents a schematic view in section of the low-pressure shaft and of the venting pipe of  FIG. 12 ; 
         FIG. 15 a    represents a schematic view in section of the upstream end-piece of the venting pipe of  FIG. 12 , mounted in the low-pressure shaft; 
         FIG. 15 b    represents a schematic view in section of the downstream part of the venting pipe of  FIG. 12 , mounted in the low-pressure shaft; 
         FIG. 16  represents a schematic perspective view of the pipe of  FIG. 12 ; 
         FIG. 17  represents a schematic perspective view, partially in transparency, of a venting pipe according to a seventh embodiment of the invention; 
         FIG. 18  represents a schematic view in cross section of the pipe of  FIG. 17 ; 
         FIG. 19  represents a partial schematic view in longitudinal section of the pipe of  FIG. 17 ; 
         FIG. 20  represents a schematic view in cross section of a venting pipe according to an eighth embodiment of the invention; 
         FIG. 21  represents a schematic view in section of the pipe of  FIG. 20  and 
         FIG. 22  represents a view in longitudinal section of the abutment region of the segments of a venting pipe according to a ninth embodiment of the invention, the lower part of  FIG. 21  showing this abutment region before compression of the deformable means and the upper part of  FIG. 21  showing this abutment region after compression of the deformable means. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 11 , a turbojet  1  according to a first embodiment of the invention comprises, in the conventional way, a fan S, a low-pressure compressor  1   a , a high-pressure compressor  1   b , a combustion chamber  1   c , a high-pressure turbine  1   d , a low-pressure turbine  1   e  and an exhaust nozzle  1   h . The high-pressure compressor  1   b  and the high-pressure turbine  1   d  are connected by a high-pressure shaft  1   f  and form therewith a high-pressure core. The low-pressure compressor  1   a  and the low-pressure turbine  1   e  are connected by a low-pressure shaft  2  and form therewith a low-pressure core. The turbojet  1  has, substantially at the upstream end of the high-pressure body, an “upstream compartment” E 1  containing components of the rolling bearing and gearing type and, substantially at the downstream end of the high-pressure body, a “downstream compartment” E 2  containing components of the rolling bearing type. 
     The low-pressure shaft  2  extends along an axis A which is the overall axis of the turbojet  1 . In the rest of the description, the concepts of longitudinal or radial relate to this axis A. 
     Referring to  FIG. 1 , the low-pressure shaft  2  is hollow. It comprises a wall  3  with an internal surface  4  and an external surface  5 . Over a majority of its length, its wall  3  has a cylindrical shape, i.e. its internal surface  4  and its external surface  5  each have a constant radius; in the case in point, the radii of the internal  4  and external  5  surfaces are constant over the entire central portion of the shaft  2 , apart from its ends. 
     Referring to  FIG. 2 , at its downstream end the low-pressure shaft  2  comprises a portion whose diameter increases rapidly in the downstream direction and ends in a flange  6  for fastening to a flange  7  connecting the journal  28  of the low-pressure turbine  1   e  to the low-pressure shaft  2 , in a known fashion. 
     Inside the low-pressure shaft  2  and concentrically therewith extends a venting pipe  8 , the function of which is to guide downstream the gas flows coming from the upstream compartment E 1  of the turbojet, this function being known, as explained in the introduction. 
     The venting pipe  8  extends along the axis A of the turbojet  1 . It is hollow and has symmetry of revolution, in the case in point with an overall cylindrical shape. 
     It comprises a plurality of segments, in the case in point two segments  8   a ,  8   b , an upstream segment  8   a  and a downstream segment  8   b . Each of its segments  8   a ,  8   b  is hollow and comprises a wall  9   a ,  9   b  with an internal surface  10   a ,  10   b  and an external surface  11   a ,  11   b . The segments  8   a ,  8   b  are arranged in order to be assembled with one another, in the case in point at the central part of the low-pressure shaft  2 . 
     Each segment  8   a ,  8   b  ( FIG. 1 ) comprises an upstream end portion  12   a ,  12   b , a central portion  13   a ,  13   b  and a downstream end portion  14   a ,  14   b . The central portion  13   a ,  13   b  of each of the segments  8   a ,  8   b  is in this case cylindrical and regular, only the upstream  12   a ,  12   b  and downstream  14   a ,  14   b  end portions having particular shapes for assembly with another segment or with the low-pressure shaft  2 . 
     More precisely, the upstream segment  8   a  comprises, at its downstream end, a downstream end skirt  14   a  (forming its downstream end portion  14   a ) with a diameter slightly greater than the diameter of its central part  13   a ; more precisely, the internal surface  10   a  of the wall  9   a  of the upstream segment  8   a  at the skirt  14   a  has a diameter greater than its diameter in the central part  13   a  of the segment  8   a . The thickness of the wall  9   a  of the segment  8   a  at the skirt  14   a  is also slightly greater than its thickness in the central part  13   a  of the segment  8   a.    
     The upstream end portion  12   b  of the downstream segment  8   b  has a thickness greater than that of its central part  13   b . The downstream segment  8   b  furthermore comprises a radial edge  15   b  at the downstream end of its upstream end portion  12   b.    
     The upstream  8   a  and downstream  8   b  segments are arranged in order to be assembled with one another in longitudinal alignment, i.e. abutting one another and more precisely sleeved at their downstream  14   a  and upstream  12   b  end portions, respectively. In the case in point, the upstream end portion  12   b  of the downstream segment  8   b  is sleeved into the downstream end skirt  14   a  of the upstream segment  8   a , the diameter of the internal surface  10   a  of the wall  9   a  of the upstream segment  8   a  at its skirt  14   a  being substantially equal to (slightly greater than) the diameter of the external surface  11   b  of the wall  9   b  of the downstream segment  8   b  at its upstream end portion  12   b.    
     A ring  16  of deformable material is mounted at the interface between the segments  8   a ,  8   b . It is in this case a ring of elastomer, for example an elastomer of the fluorocarbon type (for example of category 64C8, 64C6 or 60C7), an elastomer of the nitrile type (for example of category 21A7 or 21A8), an elastomer of the ethylene-propylene type (for example of category 41B8) or an elastomer of the polyurethane type. The material forming the ring  16  is selected as a function of its mechanical characteristics (deformation, hardness, thermal stability), its compatibility with various fluids (such as the synthetic oil and the fuel) and its resistance to atmospheric agents. Other materials satisfying the constraints defined by the person skilled in the art may of course be suitable, if they are deformable. 
     The ring  16  has an internal surface  17 , an external surface  18 , an upstream surface  19  and a downstream surface  20 . In the case in point, the ring  16  is preformed so that its external surface  18  has a convex shape. More precisely in the case in point, the ring  16  is sleeved over the upstream end portion  12   b  of the downstream segment  8   b , its downstream surface  20  bearing on (and in the case in point adhesively bonded to) the upstream surface of the edge  15   b . The ring  16  may be mounted with force on the downstream segment  8   b  or mounted with clearance, with or without adhesive bonding. 
     The ring  16  is arranged so that, before assembly of the segments  8   a ,  8   b  with one another, its external surface  18  has a radius R 1  less than the radius R 2  of the internal surface  4  of the low-pressure shaft  2 , in the case in point a radius R 1  substantially equal to (slightly greater than in the central part) the external radius of the edge  15   b . In other words, the ring  16  is arranged in order to be mounted with clearance in the low-pressure shaft  2 . The ring  16  is arranged in order to be deformed during the assembly of the segments  8   a ,  8   b  with one another in order to bear on the low-pressure shaft  2  and thus form a supporting portion of the venting pipe  8  on the low-pressure shaft  2 . More precisely, the ring  16  is arranged so that its external surface  18  bears on the internal surface  4  of the low-pressure shaft  2 . This deformation results from the compression of the ring  16  when the upstream segment  8   a  is sleeved onto the downstream segment  8   b , the radii of their sleeved portions (downstream portion  14   a  of the upstream segment  8   a  and upstream portion  12   b  of the downstream segment  8   b ) being shaped so that their sleeving by relative longitudinal translation is possible only by displacing the upstream surface  19  of the ring  16  in translation downstream, the downstream surface  20  for its part being blocked in longitudinal translation by the radial edge  15   b , which imparts radial deformation to the ring  16 . In other words, the ring  16  is radially deformed because of a longitudinal compression force of the ring  16  between the segments  8   a ,  8   b  and more precisely between the downstream end of the upstream segment  8   a  and the edge  15   b  of the downstream segment  8   b.    
     The method for mounting the venting pipe  8  in the low-pressure shaft  2  will now be described in more detail, with reference more particularly to  FIGS. 2 to 5 . 
     Referring to  FIG. 2 , in a first step, the downstream segment  8   b  is mounted in the low-pressure shaft  2 , via the downstream part thereof. The downstream segment  8   b  comprises at least one sealing means, in the case in point two circumferential seals  24   a ,  24   b  arranged in order to bear on corresponding zones of the internal surface  4  of the low-pressure shaft  2 . More precisely, the second seal  24   b  bears on the journal  28  of the low-pressure turbine  1   e  connected to the low-pressure shaft  2  by means of the fastening flanges  6 ,  7 . The downstream segment  8   b  comprises a circumferential stop rib  25  arranged in order to abut on the journal  28  of the low-pressure turbine  1   e . By virtue in the case in point of a system  25   a  comprising pins and recesses, the abutment of the rib  25  on the journal  28  of the low-pressure turbine  1   e  makes it possible to fulfill an antirotation function, that is to say to prevent rotation of the downstream segment  8   b  relative to the low-pressure shaft  2 , by means of the journal  28  of the low-pressure turbine  1   e . Furthermore, the downstream segment  8   b  is blocked in translation relative to the low-pressure shaft  2  by a nut  28 ′ which blocks its stop  25  in translation in the downstream direction. 
     In a second step, referring to  FIG. 3 , the upstream segment  8   a  is sleeved via the upstream end of the low-pressure shaft  2  and translated in the direction of the downstream segment  8   b , as indicated by the arrow F. More precisely in the case in point, after the downstream segment  8   b  has been mounted in the low-pressure shaft  2 , the upstream segment  8   a  is inserted by translation into the low-pressure shaft  2  and, during this translation, its downstream end  14   a  approaches the end  12   b  of the downstream segment  8   b.    
     In a third step, referring to  FIG. 4 , the segments  8   a ,  8   b  are brought further toward one another by relative translation and the downstream end of the downstream end portion  14   a  of the upstream segment  8   a  comes in contact with the upstream surface  19  of the ring  16 . 
     In a fourth step, referring to  FIG. 5 , the translation is continued and the downstream end of the downstream end portion  14   a  of the upstream segment  8   a  bears on the ring  16  (and more precisely on its upstream surface  19 ), the effect of which is to radially deform the ring  16 , of which the radius of the external surface  18  consequently increases, as explained above. The translation of the segments  8   a ,  8   b  with respect to one another is continued until the external surface  18  of the ring  16  bears against the internal surface  4  of the low-pressure shaft  2 , as can be seen in  FIG. 5 , this bearing being dimensioned in order to be exerted along a surface sufficient to fulfill the function which is assigned to it; in the case in point, the position of the segments  8   a ,  8   b  with respect to one another (and therefore the compression of the ring  16 ) is regulated by abutment of the upstream end of the upstream portion  12   b  of the downstream segment  8   b  on a corresponding supporting surface of the downstream end of the downstream portion  14   a  of the upstream segment  8   a ). 
     In order to facilitate and guide the translation of the upstream  8   a  and downstream  8   b  segments with respect to one another, a guiding tool may be used, for example an internal tube having the same diameter as the smallest internal diameter of the upstream  8   a  and downstream  8   b  segments, the guiding tool being withdrawn after mounting of the upstream  8   a  and downstream  8   b  segments. 
     The segments  8   a ,  8   b  are arranged in order to be secured in rotation after their assembly; to this end, they have means for securing in rotation. In the case in point, the downstream end portion  14   a  of the upstream segment  8   a  comprises a pin  21   a  arranged in order to be accommodated in a notch  21   b  of the upstream end portion  12   b  of the downstream segment  8   b , in order to secure the segments  8   a ,  8   b  in rotation. The pin  21   a  is in the case in point fastened in an adapted housing of the downstream end portion  14   a  of the upstream segment  8   a . In order to be able to assemble the segments  8   a ,  8   b  together, and more precisely sleeve their downstream  14   a  and upstream  12   b  end portions with one another, the antirotation pin  21   a  and its housing notch  21   b  must be aligned, so that the pin  21   a  can be received in the notch  21   b , without which the pin  21   a  prevents any translation movement of the segments  8   a ,  8   b  toward one another once it is in contact with the upstream end of the upstream portion  12   b  of the downstream segment  8   b . Thus, during the third (or fourth) step, the downstream segment  8   b  is, if necessary, driven in rotation about its axis A in order to align the pin  21   a  and the notch  21   b . In the case in point, in the representation of  FIG. 4  (start of the contact between the downstream end of the upstream segment  8   a  and the upstream surface  19  of the ring  16 ), the pin  21   a  has not yet started to be inserted into its housing notch  21   b  but is in proximity thereto; it is therefore during the fourth step described above that the pin  21   a  is inserted into its housing  21   b.    
     According to an embodiment which is not represented, the downstream segment  8   b  comprises a plurality of notches  21   b ; it is thus easier to align the pin  21   a  with a notch  21   b ; notches  21   b  may be distributed over the entire periphery of the segment  8   b  or only over a portion thereof. 
     Referring to  FIG. 6 , once the desired position for the upstream segment  8   a  has been reached, the latter is blocked in translation with respect to the low-pressure shaft  2  by virtue of a nut  23  fastened to its upstream end. This axial blocking nut  23  may also fulfill a function of blocking in rotation. It will be noted in  FIG. 6  that the upstream portion  12   a  of the upstream segment  8   a  comprises at least one sealing means (in the case in point three circumferential seals  22   a ,  22   b ,  22   c ) bearing on corresponding zones of the internal surface  4  of the low-pressure shaft  2 . More precisely, the function of the seal  22   b  is to avoid introduction of oil or oil-laden air into the internal cavity  4  of the upstream region of the low-pressure shaft  2 . 
     Alternative embodiments will be described with reference to  FIGS. 7 to 22 . In these embodiments, the same numerical references are used for elements with a structure or function which is identical, equivalent, similar or comparable to those of the elements of the turbojet of  FIGS. 1 to 6 , in order to simplify the description. Furthermore, not all of the description of the venting pipe of  FIGS. 1 to 6  is necessarily repeated, this description applying to the venting pipe of  FIGS. 7 to 22  when there are no incompatibilities. Only the significant structural and functional differences will be described. 
     Referring to  FIG. 7 , according to a second embodiment, the upstream segment  8   a  and the downstream segment  8   b  are secured in rotation by virtue of longitudinal splines  21   a ′,  21   b ′ respectively arranged on the downstream  14   a  and upstream  12   b  portions of these segments  8   a ,  8   b . These splines  21   a ′,  21   b ′ are enmeshed in a manner known per se in order to secure the two segments  8   a ,  8   b  in rotation. 
     Of course, other means for securing the segments  8   a ,  8   b  in rotation and/or in translation with one another may be envisioned. For example, their downstream  14   a  and upstream  12   b  ends could be threaded and screwed to one another, in which case blocking in rotation is furthermore ensured. 
     Referring to  FIG. 8 , according to a third embodiment, a rigid intermediate ring  26  is mounted between the deformable ring  16  and the downstream end  14   a  of the upstream segment  8   a ; such a rigid ring  26  makes it possible to hold the deformable ring  16  in position. It is, for example, adhesively bonded to the deformable ring  16  on their contact faces. The rigid ring  26  slides over the downstream segment  8   b  during the displacement of the upstream segment  8   a  and the compression of the deformable ring  16 , the upstream segment  8   a  transmitting its compression forces to the ring  16  via the rigid ring  26 . 
     Furthermore, in this embodiment, in proximity to the upstream end of its upstream end portion  12   b , the downstream segment  8   b  comprises a sealing joint  27  housed in a groove and arranged in order to be compressed radially between the upstream segment  8   a  and the downstream segment  8   b  in order to avoid a possible gas leak in a possible clearance between these two segments  8   a ,  8   b.    
     Referring to  FIG. 9 , according to a fourth embodiment, the segments  8   a ,  8   b  comprise means for securing in rotation of the pin/notch type as in the embodiment of  FIGS. 1 to 6 , although the dimensioning of the various elements is such that the pin  21   a  is engaged in the notch  21   b  before compression of the deformable ring  16 . The benefit is that the angular position of the two segments  8   a ,  8   b  with respect to one another is easier to achieve and is not interfered with by the compression of the ring  16  (which could hinder the rotation of the segments  8   a ,  8   b  with respect to one another). 
     Referring to  FIG. 10 , according to a fifth embodiment, the segments  8   a ,  8   b  are not secured in rotation directly to one another but are secured in rotation with the low-pressure shaft  2  by independent means. Thus, in the case in point, the downstream segment  8   b  is secured in rotation with the low-pressure shaft  2  by a system  25   a  of pins and a recess at its abutment flange  25  (as described with reference to  FIG. 2 ) while the upstream segment  8   a  is secured in rotation by virtue of adapted means  29  at its upstream end, in the case in point an arrangement of pins and recesses (as described above) or catches and notches at the abutment flange of its upstream end. One benefit of not having means for blocking the segments  8   a ,  8   b  in rotation at their interface is that these segments  8   a ,  8   b  can be mounted “blind”, that is to say without paying attention to their respective angular positions. 
     The invention has been presented in the preceding embodiments in relation to a venting pipe  8  formed by two segments  8   a ,  8   b . Of course, the venting pipe  8  may comprise a number of segments  8   a ,  8   b  greater than two, in which case a point of bearing with the low-pressure shaft  2  may be formed at the interface between each pair of successive segments. The selected number of segments depends in particular on the length of the venting pipe  8  and the desired number of supports on the low-pressure shaft  2 . The blocking of the various segments in rotation with respect to the low-pressure shaft  2  may be carried out by blocking the successive segments in rotation with respect to one another and/or directly between some (or all) segments and the low-pressure shaft  2 . 
     Furthermore, the invention has been presented with mounting of the downstream segment  8   b  before the upstream segment  8   a . Depending on the structure of the turbojet, this order may be reversed. 
     The invention has been presented with a deformable ring  16  mounted on the downstream segment  8   b  of the venting pipe  8 , although it is clear that it may be mounted on the upstream segment  8   a.    
     Referring to  FIG. 12 , a sixth embodiment of the invention is presented in which the segments of the pipe  8  are arranged so that they can be preassembled before they are mounted in the low-pressure shaft  2 . In the embodiment described, the venting pipe  8  comprises three segments, an upstream segment  8   a , a downstream segment  8   b  and an intermediate segment  8   c  which extends between the upstream  8   a  and downstream  8   b  segments; this embodiment may of course be envisioned with two segments or more than three segments. 
     As for the preceding embodiments, each of its segments  8   a ,  8   b ,  8   c  comprises an upstream end portion  12   a ,  12   b ,  12   c , a central portion  13   a ,  13   b ,  13   c  and a downstream end portion  14   a ,  14   b ,  14   c . Each of its segments  8   a ,  8   b ,  8   c  is hollow and comprises a wall  9   a ,  9   b ,  9   c  with an internal surface  10   a ,  10   c  and an external surface  11   a ,  11   c  (only the surfaces of the upstream  8   a  and intermediate  8   c  segments are referenced in the figures). The central portion  13   a ,  13   b ,  13   c  of each of the segments  8   a ,  8   b ,  8   c  is in this case cylindrical and regular in shape, only the upstream  12   a ,  12   b ,  12   c  and downstream  14   a ,  14   b ,  14   c  end portions having particular shapes for assembly with another segment or the low-pressure shaft. 
     The upstream  8   a  and intermediate  8   c  segments are arranged in order to be assembled with one another in longitudinal alignment, i.e. abutting with one another and more precisely sleeved at their downstream  14   a  and upstream  12   c  end portions, respectively. The intermediate  8   c  and downstream  8   b  segments are arranged in order to be assembled with one another in the same way, at their downstream  14   c  and upstream  12   b  end portions, respectively. 
     As above, a ring  16  of deformable material is mounted at each of the interfaces between the pairs of segments ( 8   a ,  8   c ), ( 8   c ,  8   b ). 
     According to the particular characteristic of this sixth embodiment, the segments  8   a ,  8   b ,  8   c  are arranged so that they can be assembled with one another before they are mounted in the low-pressure shaft  2 . Prior assembly of the segments  8   a ,  8   b ,  8   c  is intended to mean that the segments  8   a ,  8   b ,  8   c  are assembled beforehand, i.e. connected to one another, in relative positions corresponding substantially to their positions in operation, except for the fact that the rings  16  are not yet (fully) deformed; slight prior deformation may be envisioned so long as it does not prevent mounting of the pipe  8  in the low-pressure shaft  2 . The rings  16  are deformed during the mounting of the pipe  8  in the low-pressure shaft  2  in order to form supports on this shaft  2 . By virtue of the preassembly, it is possible to handle the pipe  8 , comprising its three pipe segments  8   a ,  8   b ,  8   c , straightforwardly and in a single unit. 
     The abutment region of the upstream segment  8   a  and of the downstream segment  8   c  will now be described with reference to  FIG. 13 . This description applies mutatis mutandis to the abutment region between the intermediate segment  8   c  and the downstream segment  8   b , these abutment regions being similar in the case in point. 
     More precisely, referring to  FIG. 13 , the upstream segment  8   a  comprises, at its downstream end, a downstream end skirt  14   a  (forming its downstream end portion  14   a ) with a diameter slightly greater than the diameter of its central part  13   a ; more precisely, the internal surface  10   a  of the wall  9   a  of the upstream segment  8   a  at the skirt  14   a  has a diameter greater than its diameter in the central part  13   a  of the segment  8   a . The thickness of the wall  9   a  of the segment  8   a  at the skirt  14   a  is also slightly greater than its thickness in the central part  13   a  of the segment  8   a.    
     Furthermore, the upstream end portion  12   c  of the intermediate segment  8   c  has a thickness greater than that of its central part  13   c . The intermediate segment  8   c  furthermore comprises a radial edge  15   c  at the downstream end of its upstream end portion  12   c.    
     The upstream end portion  12   c  of the intermediate segment  8   c  is sleeved into the downstream end skirt  14   a  of the upstream segment  8   a , the diameter of the internal surface  10   a  of the wall  9   a  of the upstream segment  8   a  at its skirt  14   a  being substantially equal to (slightly greater than) the diameter of the external surface  11   c  of the wall  9   c  of the intermediate segment  8   c  at its upstream end portion  12   c.    
     A ring  16  of deformable material is mounted at the interface between the segments  8   a ,  8   c . The ring  16  has an internal surface  17 , an external surface  18 , an upstream surface  19  and a downstream surface  20 . In the case in point, the ring  16  is preformed so that its external surface  18  has a convex shape. More precisely in the case in point, the ring  16  is sleeved over the upstream end portion  12   c  of the intermediate segment  8   c , its downstream surface  20  bearing on (and in the case in point adhesively bonded to) the upstream surface of the edge  15   c . The ring  16  may be mounted with force on the intermediate segment  8   c  or mounted with clearance, with or without adhesive bonding. 
     The upstream  8   a  and intermediate  8   c  segments are assembled with one another with the aid of pins  31 , in the case in point three in number regularly distributed angularly. Each pin  31  is secured to the downstream skirt  14   a  of the upstream segment  8   a  and is, to this end, mounted with force in an orifice thereof. It is furthermore received in a housing forming a slideway  32 , formed in the opposing surface of the upstream portion  12   c  of the intermediate segment  8   c ; this slideway  32  allows the segments  8   a ,  8   c  to slide with respect to one another but only in the longitudinal dimension of the slideway  32 . Thus, by virtue of the pins  31 , the segments  8   a ,  8   c  are assembled with one another, blocked in rotation with respect to one another and free to slide with respect to one another but only along an excursion corresponding to the length of the slideway  32 . A sealing joint  27  is housed in a groove and arranged in order to be compressed radially between the upstream segment  8   a  and the intermediate segment  8   c  in order to avoid a possible leak of gas in a possible clearance between these two segments  8   a ,  8   c.    
     The pipe  8  with its assembled segments  8   a ,  8   b ,  8   c  can be handled as a unitary object of which the various components are connected to one another, the only degree of freedom being the longitudinal translation between the segments  8   a ,  8   b ,  8   c , but only in the dimension of the slideway  32 . The handling of the pipe  8  as a single unit is therefore easy, which facilitates its mounting in the low-pressure shaft  2 . 
     As can be seen in  FIG. 13 , when the pipe  8  is threaded into the shaft  2 , the rings  16  are not compressed, their exterior surfaces  18  therefore having a maximum exterior diameter less than the diameter of the internal surface  4  of the low-pressure shaft  2 . Subsequently, during its mounting in the shaft  2 , the pipe  8  is longitudinally constrained which causes the segments  8   a ,  8   b ,  8   c  to approach one another and compress the rings  16  at their interfaces, these rings  16  thus forming supports on the internal surface  4  of the low-pressure shaft  2 , as in the preceding embodiments. 
     As previously, circumferential joints are arranged in order to bear on corresponding zones of the internal surface  4  of the low-pressure shaft  2 , and more precisely in particular on the journal  28  of the low-pressure turbine  1   e  connected to the low-pressure shaft  2  by means of the fastening flanges  6 ,  7 . 
     It will furthermore be noted in the case in point that the pipe  8  is fastened on its upstream side (which corresponds to the upstream portion  12   a  of the upstream segment  8   a ) to an upstream end-piece  33  intended to be fastened on the upstream side of the low-pressure shaft  2 . The end-piece  33  comprises antirotation means (for example lugs)  34  arranged in order to cooperate with corresponding means (for example housings) of the low-pressure shaft  2  in order to fix the angular position of the end-piece  33  with respect to the low-pressure shaft  2 . The end-piece  33  furthermore comprises sealing joints  35 ,  36 , one  35  on the upstream side of the end-piece  33  and the other  36  on its downstream side, on either side of openings  37  allowing the gases G coming from the upstream oil compartment E 1  of the turbojet  1  to pass through. The end-piece  33  is fastened to the low-pressure shaft  2  with the aid of an upstream nut  23 . 
     It will be noted in passing that the end-piece  33  is in this case a separate piece of the pipe  8 , in contrast to the embodiments described with reference to  FIGS. 1 to 11  in which this end-piece is formed directly by the upstream segment of the pipe  8 , integral therewith. 
     The upstream portion  12   a  of the upstream segment  8   a  is sleeved over the downstream portion of the upstream end-piece  33 , the end of the upstream portion  12   a  of the upstream segment  8   a  comprising mounting bosses  39  making it possible to consolidate the position of all the elements, by placing the upstream segment  8   a  in pressure between the low-pressure shaft  2  and the upstream end-piece  33 ; the radial dimension of these bosses  39  is 0.56 mm in the case in point. A sealing joint  38  is provided between the upstream end-piece  33  and the upstream segment  8   a.    
     The mounting method is, in a simplified way, as follows:
         the segments  8   a ,  8   b ,  8   c  of the pipe  8  are assembled with one another;   the upstream end-piece  33  is mounted from the upstream side of the low-pressure shaft  2  and fastened in position by virtue of the upstream nut  23 ;   the pipe  8  is mounted from the downstream side of the low-pressure shaft  2 , the segments  8   a ,  8   b ,  8   c  being forced to slide with respect to one another in the dimension of the slideways  32  (in the case in point with identical lengths) in order to compress the rings  16  and deform them so that they form points of bearing on the low-pressure shaft  2 ;   the pipe  8  is fastened in position by its downstream end, allowing the mounting to be completed.       

     On the downstream side, the fastening preferably takes place by virtue of antirotation means, in the case in point lugs  45 , of the downstream portion  14   b  of the downstream segment  8   b , which are arranged in order to cooperate with antirotation means, in the case in point corresponding housings  46 , of the low-pressure shaft  2 . 
     The displacement of the segments  8   a ,  8   b ,  8   c  with respect to one another is obtained in the case in point by virtue of a downstream nut  47  making it possible, by screwing it, to push the downstream segment  8   b  of the pipe  8  in the upstream direction and therefore compress the rings  16  against the low-pressure shaft  2 . Once the downstream nut  47  has been fully screwed, the assembly is fixed in position, the antirotation lugs  45  being blocked in the corresponding recesses  46  of the pressure shaft  2 . 
     The various antirotation elements provided on the various pieces of the assembly make it possible to avoid any risk of twisting these pieces, in particular the segments  8   a ,  8   b ,  8   c.    
     Referring to  FIGS. 17 to 19 , a means equivalent to the pins  31  of the embodiment of  FIGS. 12 to 16  is described. In this embodiment, transverse rods  40  are fixed with force into corresponding housings of the downstream portion  14   a  of the upstream segment  8   a  (or of the downstream portion  14   c  of the intermediate segment  8   c ), these rods  40  being arranged in order to be received in housings  41  forming slideways, which are formed in the upstream portion  12   c  of the intermediate segment  8   c  (or in the upstream portion  12   a  of the downstream segment  8   b ). As previously, the cooperation of the rods  40  with their housings  41  makes it possible to secure the segments  8   a ,  8   c  in rotation and in translation with freedom of movement in the longitudinal dimension of the housings forming slideways  41 . In the case in point, the pipe  8  comprises two diametrically opposite rods  40  at each interface between two segments ( 8   a ,  8   c ), ( 8   c ,  8   b ); a single rod or more than two rods could be provided. 
     Referring to  FIGS. 20 and 21 , another means equivalent to the pins  31  of the embodiment of  FIGS. 12 to 16  is described. In this embodiment, a toric rod  42  is fixed in a corresponding peripheral housing of the downstream portion  14   a  of the upstream segment  8   a  (or of the downstream portion  14   c  of the intermediate segment  8   c ), this rod  42  being arranged in order to be received in a housing  43  forming a slideway, which is formed in the upstream portion  12   c  of the intermediate segment  8   c  (or in the upstream portion  12   b  of the downstream segment  8   b ). The toric rod  42  is rolled up in its housing through an orifice  44  and extends circumferentially (and therefore circularly) once in position (it is seen while being rolled up in  FIG. 20 ). The cooperation of the rod  42  with its housing  43  makes it possible to secure the segments  8   a ,  8   c  in translation with freedom of movement in the longitudinal dimension of the slideway  41 . Prevention of rotation of the segments  8   a ,  8   c  with respect to one another is ensured in the case in point when all of the venting pipe  8  and the end-piece  33  are permanently secured by the upstream  23  and downstream  47  nuts, so that the deformable ring  16  bears on the internal surface  4  of the turbine shaft  2 , thus blocking the degree of rotation of the pipes  8   a ,  8   c.    
     The embodiment of  FIG. 22  is identical to that of  FIGS. 12 to 16 , with the only difference that the deformable means is formed not from elastomer but from deformable metallic material. In the case in point, it is a metallic bearing joint  16 ′, which in the case in point is in the form of a convex annular metal plate  16 ′; this metallic bearing joint  16 ′ thus has a hollow annular shape and comprises a convex wall between two annular curved edges  16 ′ a ,  16 ′ b . Under the effect of a longitudinal constraint (between adjacent segments ( 8   a ,  8   c ), ( 8   c ,  8   b )), the metallic bearing joint  16 ′ deforms radially (changing from its shape at the bottom of  FIG. 22  to its shape at the top of  FIG. 22 ) in order to form a radial support on the low-pressure shaft. Since it is metallic, the metallic bearing joint  16 ′ can withstand high temperatures; it may for example be formed from nickel alloy such as Inconel X750 (registered trademark), which can withstand temperatures of the order of 500 or 600° C. Such an embodiment of the deformable means  16 ,  16 ′ between two segments ( 8   a ,  8   b ), ( 8   a ,  8   c ), ( 8   b ,  8   c ) may of course be applied to all the embodiments described. The selection of the material of the rings  16 ,  16 ′ makes it possible to cover a wide range of possible working temperature, from a cold temperature to a temperature in the case in point substantially equal to 600°. 
     In the various embodiments, the various toric bearing joints for the segments  8   a ,  8   b ,  8   c  of the pipe  8  or its upstream end-piece  33  may also be replaced by metallic bearing joints, formed for example from cast-iron, making it possible to cover temperatures up to about 600° C. 
     The invention has been described with reference to preferred embodiments, although it is clear that other embodiments may be envisioned. 
     In particular, the characteristics of the various embodiments described may be combined together, if they are not incompatible.