Patent Publication Number: US-6986482-B2

Title: Suspension part of a turbojet engine

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The subject of this invention is a suspension part for mounting a turbojet engine. 
   2. Discussion of the Background 
   The subject of this invention is a suspension part for mounting a turbojet engine. 
   Reference will be made in particular to French patent 2 680 353 to uncover an existing example of such a suspension. It comprises an elongate part known as a fitting, or coat hook, on account of its shape, that the invention sets out to improve and that runs between a fixed strut or similar structure and the turbojet engine that is to be supported. Four fixing elements comprising screws and barrel nuts are arranged at the corners of the fitting to connect it to the fixed structure. This fitting runs transversely and carries connecting rods at its ends, which rods are articulated also to the turbojet engine; a third connecting rod runs between the center of the fitting and a portion of the turbojet engine to provide backup support should one of the end connecting rods break. 
   However, the fitting has itself to be protected against breakage, because the dropping-off of the turbojet engine would then be more probable than it would with the breakage of a connecting rod. It needs in particular to withstand the vertical and transverse forces and moments about the axis of the turbojet engine under varying service conditions, including when a turbojet engine blade becomes detached and significant dynamic imbalance arises. This situation produces forces that are considerable, but can be estimated fairly well in order to engineer the fitting. The phenomenon perhaps the most worthy of concern is, however, fatigue in normal service, which may give rise to cracks, the propagation of which ends up sectioning the fitting even under light load; it is not really practical to seek to guard against this cracking by strengthening the fitting as the desire is, on the contrary, to keep to a modest weight and bulk. 
   SUMMARY OF THE INVENTION 
   The concept underlying the invention is therefore to engineer the fitting in such a way that it withstands extreme static forces without reinforcements being added to prevent the onset of fatigue cracks; however, a means is provided for limiting their propagation by making sure that they never section the fitting into two comparable pieces but that they can render unserviceable just one of the four fixing elements for fixing to the strut. This then yields a situation in which the turbojet engine is supported by the remaining three elements, that is to say under normal conditions, on one side and with a single fixing element to react the forces on the corresponding connecting rod on the other side. The latter element would then be subjected to greater mechanical stresses, but ones which would remain lower than the extreme stresses designed for, and the fitting would essentially hold together. 
   This attitude differs completely from the one that led to the proposal described in the aforementioned patent and which consisted in splitting the fitting into two parallel elements in order to continue to provide support should one of them break. The consequence was either that the fitting was made heavier if each element had to withstand the static forces or that there was weakening as a result of the split. 
   To sum up, the invention relates to a suspension part for mounting a turbojet engine arranged horizontally between a fixed structure and at least two connecting rods articulated to the turbojet engine and comprising two ends equipped with two elements, juxtaposed in a widthwise direction of the part, for fixing to the fixed structure, the ends being shaped into two pairs of yokes respectively flanking the connecting rods and each comprising one of the fixing elements and a bearing of a journal of a connecting rod which bearing is situated underneath said fixing element, characterized in that slots passing through the ends of the part run between the yokes without running over a central portion of the part. 
   Advantageously, the yokes are thicker at the fixing elements than they are at the bearings of the journal and have an additional thickness over the connecting rods. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described in conjunction with the single FIGURE which is a perspective view of the suspension part. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The suspension part or fitting, bearing the reference  1 , runs between a strut  2  which is a fixed airplane structure and a rear section of turbojet engine, the casing  3  of which has been sketched out. It is arranged horizontally and has an elongate shape; it mainly comprises a top plate  4  and pairs of yokes  5  which lie at its ends; longitudinal ribs  6  connect the corresponding yokes  5  together and a triangular support  7  is also arranged under the plate  4 . The pairs of yokes  5  carry respective connecting rods  8  and  9  for supporting the turbojet engine  3 , to which they are articulated by their bottom ends. A vertical yoke or triangular support  7  is articulated to a pair of central yokes  26  of the turbojet engine  3  by a spindle  25  mounted with clearance. At each of the corners of the suspension part  1  there is a fixing element  11  allowing it to be supported by the strut  2  and which comprises a screw  12  the end of which is engaged in a barrel nut  13  (of half-moon cross section) arranged horizontally through a drilling  17  in a top portion  15  of the corresponding yoke  5 ; the yokes  5  also have a lower portion  16  through which there is made a drilling  17  to house a journal  18  of the connecting rod  8  or  9 . 
   One essential feature of the invention, is a slot  19  running across each of the ends of the top plate  4  between the yokes  5  and running over a portion of the plate  4 , or at least as far as the end of the yokes  5 , in the main direction of elongation of the part  1 . The fixing elements  11  that are adjacent and juxtaposed in the lengthwise direction of the part  1  are therefore separated. However, it is in accordance with the invention that the slots  19 , although being directed toward one another, should not meet, so that the suspension part  1  remains a single part and does not therefore need to be made heavier with connecting bolts or similar elements. The technical role of the slots  19  is to limit the propagation of cracks such as the crack  20 , which may appear through fatigue because of the forces that the suspension part  1  has to transmit between the connecting rods  8  and  9  and the fixing elements  11 , to progress transversely from one long edge side of the plate  4  to the other, stopping them. When the crack  20  has reached the slot  19 , it has then separated a portion of the suspension part  1  from the remainder thereof, and generally isolated one of the fixing elements  11 , whose role of supporting the turbojet engine  3  is then reduced, especially if the crack  20  has reached the drilling  14 , which is then likely to open up; however, the other three fixing elements  11  continue to belong to one and the same compact bit of the suspension part  1  and to act as before. The dimensions of the suspension part  1  in general and of the yokes  5  in particular are chosen with that in mind, particularly the upper portions  15  of the yokes  5  have enough thickness to continue to withstand the forces of the turbojet engine  3  even if forces can continue to pass through just one of the yokes  5  of the pair. An additional thickness  21  of the top portion  15  is located on the inside of the yokes  5 , over the connecting rods  8  or  9 , without there being any need to increase the overall width of the suspension part  1 . That is why the bottom portion  16  for supporting the connecting rods  8  and  9  is placed under the top portion  15 , with the additional advantage that the moment produced by the overhang between the connecting rod  8  or  9  and the fixing element  11  that remains when the other has failed remains modest, thus avoiding introducing excessive bending stresses into the yokes  5 . 
   As can be seen in the exemplary suspension part shown in the figure, each of the slots  19  can end with an opening which is larger than other portions of the slots  19 . As also seen from the figure, the slots  19  can define openings which are narrower than the distance between the inner faces of the corresponding lower or bottom portions  16  of the yokes  5 . The openings of the slots  19  can also be narrower than the distance between the inner faces of the top or upper portions  15  of the yokes  5 . 
   The slots  19  run past the places in the suspension part  1  where there are stress concentrations and where cracks are likely to originate, that is to say over the yokes  5  and the journals  18 , and between the fixing elements  11 . The central piece of the suspension part  1  is less highly stressed and can therefore remain free of slots  19 , since there is hardly any risk of cracks developing there. It will also be noted that the-ribs  6  are in the shape of arches, low under the center of the plate  4  and tall nearer the yokes  5 . 
   To sum up, it can be seen that the invention allows the suspension part  1  to be kept in service without danger, even in the event of fatigue cracking, without there being any need to increase its bulk, its weight or its complexity.