Abstract:
A jet engine suspension comprises a master component for attachment, fixed to an aircraft pylon. If it breaks and if the situation arises where the fixing screws are no longer able to transmit force, a peg takes over: the master component collapses until an upper shoulder rests on the pylon and supports this component. The peg is free of dynamic stress in normal service and there is absolutely no risk of it cracking in fatigue.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The subject of this invention is an aircraft jet engine suspension.  
           [0003]    2. Summary of the Prior Art  
           [0004]    One common kind of suspension comprises a pair of devices for attachment to the front and rear of the jet engine, each device fixed to a pylon of the aircraft, and a pair of roughly longitudinal intermediate link rods one of whose functions is to spread the loads through the two attachment devices appropriately so that the attachment device at the rear transmits to its pylon the transverse and vertical forces and the moment about the longitudinal axis in addition to the thrust of the engine, which is exerted first and foremost on the front suspension.  
           [0005]    This attachment device at the rear is thus subjected to many static and dynamic loads which lead to a high risk of breakage. If breakage occurs therein, other damage may ensue.  
           [0006]    Fatigue cracks may thus appear on the master component of the attachment device, which runs along the pylon and toward which the longitudinal link rods and the lower links carrying the jet engine converge. This master component is generally fixed to the pylon by four screws arranged in a rectangle. If the form of fatigue progresses to the extent of breaking through the master component or of isolating some of the screws, then the forces are transmitted only by the other screws, and this is obviously highly unfavorable. The idea of splitting the master component into two parts between which the screws are distributed such that a crack stops at the split has been had.  
           [0007]    Although this arrangement does stop the master component from breaking, it entails joining together the two portions which are separated by the split using bolts which greatly increase the weight of the suspension. A suspension of this kind is described in French Patent 2 680 353, where the master component is not only split but duplicated so that each of its parts alone is able to withstand the loads: its weight is then even greater.  
           [0008]    The risk of fatigue breakage relates to the actual fixing screws. Certain precautions need to be taken such as limiting the tightening force applied to them on mounting, in order to reduce this risk.  
         SUMMARY OF THE INVENTION  
         [0009]    The object of the invention is to substitute, for screws rendered unserviceable, at least one peg passing at least partially through the master component of the attachment device and the pylon which takes over in transmitting loads. The peg is produced in such a way as not to experience significant dynamic loads beforehand, and this sets aside the risk of it having broken through fatigue.  
           [0010]    The invention relates specifically to a jet engine suspension comprising a device for attachment to a fixed structure, comprising a master component fixed to the structure, wherein the master component and the fixed structure are connected by a vertical peg exhibiting a central shoulder placed on the master component, an upper shoulder above a portion of the fixed structure, and a lower shoulder between which and the central shoulder the main component is compressed.  
           [0011]    Advantageously, the lower shoulder and the upper shoulder are formed by nuts engaged on threaded end portions of the peg, a device for preventing the nut of the upper shoulder from rotating being added.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is an overall view of the suspension;  
         [0013]    [0013]FIGS. 2 and 3 depict the rear suspension device with and without the link rods; and  
         [0014]    [0014]FIG. 4 illustrates a support peg of the attachment device. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    The jet engine  1  illustrated in FIG. 1 and only the case of which has been depicted, comprises in particular an intermediate casing  2  situated toward the front and an exhaust casing  3  situated toward the rear, borne respectively by a front attachment device  4  and by a rear attachment device  5 . Bolts, generally referenced as  6 , are used to fix the attachment devices  4  and  5  under pylons  7  and  8  or other fixed structures of an aircraft. A pair of link rods  9  and  10  connects the attachment devices  4  and  5 . They are more or less longitudinal and provided at their front ends with articulations  11  and  12  dependent on the intermediate casing  2  and at their rear ends with articulations  13  and  14  dependent on a balance beam  15 , that is to say a rocking lever the middle of which is articulated by a central pin  16  to a fitting  17  which forms the master component of the rear attachment device  5  and which, via lateral links  18  and  19 , supports the exhaust casing  3 .  
         [0016]    The link rods  9  and  10  are extended toward the fitting  17  and connected to it by additional articulations  20  and  21  situated on lugs  22  and  23  formed on the fitting  17  on each side of a central clevis  24  which bears the central pin  16  of the balance beam  15 .  
         [0017]    The articulations  13  and  14  of the link rods  9  and  10  to the balance beam  15  have a small amount of clearance but the additional articulations  20  and  21  to the fitting  17  have a greater amount of clearance which allows them to remain inactive, that is to say not to transmit load, under normal circumstances: the transmission of thrust and of the other forces is entirely through the balance beam  15 , but if the latter, its pin  16  or the central clevis  24  breaks, the link rods  9  and  10 , yielding to the thrust force, move longitudinally and the additional articulations  20  and  21  come into abutment, re-establishing the transmission of load to the fitting  17 . The same thing happens if a link rod,  10 , for example, breaks, the loads then being transmitted through the additional articulation  20  of the other link rod  9  and through the central pin  16 . If, finally, a pin of an articulation  13  or  14  to the balance beam  15  breaks, then the balance beam  15  becomes inactive but the loads are still transmitted via the additional pins  20  and  21 . In all these situations, the distribution of loads is modified only slightly by comparison with normal conditions. The assembly remains isostatic and therefore does not allow unforeseen stresses to arise.  
         [0018]    The characteristic device of the invention will now be described mainly with the aid of FIG. 4. This is a vertical peg  26  situated more or less at the middle of the fitting  17  and which serves to hold the plate  29  on the fitting against movements in the horizontal directions; however, its special shape, which will soon be described, allows it to play a useful part when a crack  27 , sketched in FIG. 2 and produced by fatigue, cuts the fitting  17  into two, passing, for example, through the drillings for two suspension screws  6  situated on the same side. The loads are then no longer transmitted to the pylon  8  by the screws, but the peg  26  lightens the load on the screws  6  which remain active, by its very presence and also by allowing a better distribution of the loads in the fitting  17 .  
         [0019]    The peg  26  has a central shoulder  28  which is partly engaged in a drilling in a plate  29  belonging to the pylon  8  and the rest of which is engaged in a counterbore  31  of a smaller-diameter drilling passing through part of the fitting  17  as far as a chamber  33 . The central shoulder  28  is engaged in the drilling  30  and the counterbore  31  with very little clearance. The peg  26  also comprises a lower shoulder and an upper shoulder, which consist of nuts  34  and  35  engaged on threaded ends of the peg  26 . While, however, the lower nut  34  is clamped against the fitting  17  at the top of the chamber  33 , the upper nut  35  is separated from the plate  29  by a small clearance that a pin  36  passing through the nut  35  and the peg  26  maintains, preventing the nut  35  from turning.  
         [0020]    When the crack  27  has made some of the screws  6  unserviceable, the fitting  17  collapses, carrying the peg  26  downward until the upper nut  35  touches the plate  29 , and this stops the collapsing movement: the peg  26  then supports the spar  17 .  
         [0021]    The central shoulder  28  is also advantageous for transmitting horizontal loads from the fitting  17  to the plate  29 , because its larger cross section means that it experiences lower stresses.  
         [0022]    The peg  26  may be bolstered by an additional peg  37 , placed not far from it at the middle of the fitting  17  and which has the appearance of a conventional peg, passing through the fitting  17  and the plate  29  and comprising upper and lower shoulders similar to those of the peg  26 , but no central shoulder. Circular and longitudinal clearances remain between the peg  37 , the fitting  17  and the plate  29 . Pegs  26  and  37  are not stressed before the crack  27  appears, except by a tensile load in the peg  26  between the central  28  and lower  34  shoulders, but this is an invariable clamping force which is therefore incapable of producing a fatigue crack, and except by a horizontal shear force at the central shoulder  28 , but this is modest because of the width thereof. If, however, the peg  26  breaks, the peg  37 , hitherto unloaded in respect of the horizontal loads by virtue of the clearance with which it is mounted, takes over from it in its function of holding the plate  29  on a fitting  17  in the horizontal directions.