Abstract:
Casing cover ( 10 ) in a jet engine, comprising two coaxial shells ( 36, 38 ) arranged one inside the other and joined fixedly by radial envelopes ( 40 ) inside which extend radial arms ( 26 ) of the casing, the cover being fastened at its downstream end to an element of the casing and bearing axially at its upstream end on another element of the casing, and the cover having, in the free state, an axial dimension (D) of less than the axial distance (L) between the points where its downstream end is fastened to the casing and the points where its upstream end bears axially on the casing, and being tensioned axially when it is mounted on and fastened to the casing.

Description:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
       [0001]    The present invention relates to a casing cover such as an exhaust casing cover in a jet engine, this cover comprising two coaxial shells arranged one inside the other and joined fixedly by radial envelopes inside which extend radial arms of the casing. 
         [0002]    This type of cover is mounted around a jet engine bearing support and thermally protects the exhaust casing from the stream of hot gas which originates from the combustion chamber and from the turbine of the jet engine and which flows between the shells of the cover. 
         [0003]    The cover is fastened at its downstream end to a flange of the bearing support by means of bolts and, at rest, bears at its upstream end on the casing so as to be able to expand freely under the effect of the rise in temperature during the operation of the jet engine. 
         [0004]    However, the thermal expansion of the cover, which is greater than that of the casing, precludes the upstream end of the cover from bearing on the casing, at least during the transient phases between the idling mode and the full-power operation of the jet engine. The cover is thus mounted in cantilever fashion on the bearing support via its downstream end, and is subjected to considerable vibration stresses which may result in the appearance of fissures or cracks. 
         [0005]    One solution to this problem would consist in modifying the geometry of the cover and/or in reinforcing it by means of stiffeners. However, this solution is not satisfactory since it is costly and leads to an increase in the mass of the cover, this being a disadvantage in the aeronautical industry. 
       SUMMARY OF THE INVENTION 
       [0006]    The object of the invention is in particular to provide a simple, effective and economic solution to these problems. 
         [0007]    To this end, the invention provides a casing cover in a jet engine, comprising two coaxial shells arranged one inside the other and joined fixedly by radial envelopes inside which extend radial arms of the casing, the cover being fastened at its downstream end to an element of the casing and bearing axially at its upstream end on another element of the casing, wherein the cover has, in the free state, an axial dimension of less than the axial distance between the points where its downstream end is fastened to the casing and the points where its upstream end bears axially on the casing, and is tensioned axially when it is mounted on and fastened to the casing. 
         [0008]    The axial tensioning of the cover when it is mounted on the casing makes it possible to compensate for the deviation between its axial thermal expansion and the axial thermal expansion of the casing in order to keep its upstream end bearing axially on the casing during the operation of the jet engine, thereby preventing the cover from being subjected to considerable vibration stresses. 
         [0009]    According to a feature of the invention, the difference between the axial dimension of the cover, in the free state, and the axial distance between the points where its downstream end is fastened and the points where its upstream end bears axially is substantially equal to the maximum value of the deviation between the axial thermal expansion of the cover and the axial thermal expansion of the casing during the operation of the jet engine. Thus, the upstream end of the cover always remains bearing axially or radially on the casing whatever the operating mode of the jet engine, this being sufficient to prevent the appearance of vibration stresses in the cover. 
         [0010]    This difference is, for example, approximately 1 to 1.2 millimeters in one particular embodiment. 
         [0011]    The cover is, for example, fastened at its downstream end to a flange of a bearing support by means of bolts and comprises, at its upstream end, an outwardly oriented radial lip, this radial lip being situated to the inside and upstream of a radial lip formed at the upstream end of a cylindrical element of the casing, the radial lip of the upstream end of the cover bearing axially on the radial lip of the casing element when the cover is mounted on the casing. 
         [0012]    The radial lip of the cover is advantageously formed on an outer shell of the cover which, as a result of thermal expansion, can bear radially on the casing element during the operation of the jet engine, thereby making it possible to keep the upstream end of the cover bearing axially and/or radially on the casing. 
         [0013]    When the cover has reached its maximum axial thermal expansion in relation to that of the casing, the radial lip of the cover is flush with the radial lip of the casing and the shell of the cover bears radially on the casing element. The upstream end of the cover thus always remains bearing on the casing whatever the operating mode of the jet engine. 
         [0014]    The invention also relates to a jet engine which comprises at least one casing cover, in particular an exhaust casing cover, as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention will be better understood and other details, features and advantages of the present invention will become apparent on reading the description given below by way of nonlimiting example and with reference to the appended drawings, in which: 
           [0016]      FIG. 1  is a schematic half-view in axial section of an exhaust casing cover according to the invention; 
           [0017]      FIG. 2  is a view on an enlarged scale of the means for fastening the cover shown in  FIG. 1 ; 
           [0018]      FIG. 3  is a view on an enlarged scale of the means for the axial bearing of the cover shown in  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]      FIG. 1  shows a cover  10  of an exhaust casing  12  of a jet engine, which is mounted around a bearing support  14  and which makes it possible to thermally protect the casing  12  from a stream of hot gas  16  originating from the combustion chamber (not shown) and from the turbine (not shown) of the jet engine. 
         [0020]    The bearing support  14  comprises a substantially frustoconical wall  18  extending downstream toward the axis  20  of the jet engine and bearing an outer race  21  of a bearing (not shown) for centering and guiding a shaft of the jet engine. The wall  18  of the bearing support comprises, at its downstream end, a flange  22  for fastening to bearing lubrication means  23  and is connected, at its upstream end, to an upstream end of a substantially cylindrical wall  24 . 
         [0021]    The exhaust casing  12  comprises nine radial arms  26  which are fastened at their internal ends to the cylindrical wall  24  of the bearing support by means of radial bolts  25  and at their external ends to a cylindrical element  28  of the casing by means of radial bolts  27 . 
         [0022]    Each radial arm  26  comprises an internal cavity  30  for the circulation of cooling air originating from a supply enclosure  32 , radially external to the casing element  28 , and discharged partly into an enclosure  34  which is radially internal to the wall  24  of the bearing support and which is delimited by this wall  24  and the frustoconical wall  18  of the bearing support. 
         [0023]    The cover  10  is in one piece and comprises two coaxial shells  36  and  38  which extend one inside the other and which are connected by nine radial envelopes  40  inside which extend the radial arms  26 . The internal shell  36  extends outside and at a distance from the wall  24  of the bearing support and the external shell  38  extends inside and at a distance from the casing element  28 . 
         [0024]    Each envelope  40  has an axially profiled shape and the radial arm  26  extends inside an upstream portion of the envelope and at a distance therefrom. 
         [0025]    The radial arm  26  comprises, upstream, holes  42  which open out toward the upstream end portion of the envelope  40 , which itself comprises, downstream, holes  44  which are oriented in the downstream direction and which open out into the flow path of the gas stream. The air which circulates in the internal cavity  30  of the radial arm  26  is partly discharged through the holes  42  and projected onto the upstream end portion of the envelope so that it can be cooled. This air then flows around the radial arm  26  in the envelope  40  and is injected into the gas stream  16  via the holes  44 . 
         [0026]    During the operation of the jet engine, the cover  10  is exposed to high temperatures which may reach approximately 700 to 800° C., and the air which circulates in the cavities  30  of the radial arms  26  has a temperature of approximately 300 to 400° C., thus resulting in considerable differential thermal expansions between the cover and the casing. 
         [0027]    The downstream end of the cover  10  is fastened to the bearing support  14  and its upstream end bears on the casing element  28  so that the cover maintains a freedom of axial expansion during operation. 
         [0028]    In the example represented, the internal shell  36  of the cover comprises at its downstream end a radially internal annular flange  50  which is clamped by means of bolts  55  between an annular flange  52 , situated upstream, of the wall  24  of the bearing support and the flanges  51 , situated downstream, of an annular covering  53  and  54  of an exhaust cone  56  ( FIG. 2 ), the exhaust cone extending in the downstream direction and being aligned with the internal shell  36  of the cover. 
         [0029]    The upstream end of the internal shell  36  is fastened by riveting to elastically deformable means  58  which are borne by the bearing support  14  and allow differential thermal expansions between the cover and the bearing support. 
         [0030]    The external shell  38  of the cover comprises, in the vicinity of its upstream end, a radially external annular lip  60  which has its downstream face bearing axially on the upstream face of an external annular lip  62  formed at the upstream end of the casing element  28  ( FIG. 3 ). The radial dimension of the lip  60  is greater than the radial distance between the external shell  38  and the cylindrical element  28 . 
         [0031]    The downstream end of the casing element  28  comprises elastically deformable means  64  bearing radially on the downstream end of the external shell  38  of the cover. 
         [0032]    In the current art, the cover has, in the free state, an axial dimension D, situated between the downstream bearing face of the radial lip  60  of the external shell and the upstream face of the flange  50  intended to be applied to the flange  52  of the bearing support, which is equal to the axial distance L between the downstream face of the flange  52  to which the flange  50  is applied and the upstream bearing face of the radial lip  62  of the casing element  28 . 
         [0033]    During operation, the cover expands axially and radially and the radial lip  60  of its external shell moves axially in the upstream direction in relation to its position in the free state, and thus no longer bears axially on the radial lip  62  of the casing element, a situation which may cause considerable vibration stresses in the cover and result in damage thereto. 
         [0034]    The invention makes it possible to solve this problem by virtue of a cover whose aforementioned axial dimension D is less than the axial distance L, thereby requiring the cover to be placed under axial tension so that it can be mounted on the bearing support. 
         [0035]    The difference between the axial dimension D and the axial distance L is substantially equal to the maximum deviation between the axial thermal expansion of the cover and the axial thermal expansion of the casing during the operation of the jet engine. When the axial thermal expansion of the cover becomes equal to this difference, the bearing face of the radial lip  60  is flush with the radial lip  62  of the casing element, but the radial thermal expansion of the cover is then such that it bears radially on the casing element  28 , this being sufficient to prevent the appearance of vibration stresses in the cover. 
         [0036]    The difference between the dimension D and the distance L is approximately 1 to 1.2 millimeters in one exemplary embodiment. 
         [0037]    The radial distance R between the external shell  38  and the casing element  28  is advantageously equal to or slightly less than the maximum radial thermal expansion of the cover so that the cover is kept bearing axially and/or radially on the casing element during operation ( FIG. 3 ). 
         [0038]    The cover  10  can be mounted on the exhaust casing  12  in the following way, for example with a vertical arrangement of the components: with the casing being placed to bear on a support, tooling is placed to bear on the upstream portion of the bearing support and a force is applied thereto along the axis of the casing so as to shift the downstream position of the downstream flange  50  of the cover over a distance of 1 to 1.2 mm. While maintaining the position, the screws  25  and  27  for fastening the arms to the outer casing shell and to the bearing support are tightened.