Patent Publication Number: US-7214030-B2

Title: Guide system for the outer pivot of a variable stator vane, for a turbojet stator

Description:
FIELD OF THE INVENTION 
   The invention relates to the control of variable stator vanes in turbojets, particularly turbojets for use in aeronautics. It is particularly applicable to the control of air inlet guide vanes in turbojet compressors, called variable stator vanes. 
   PRIOR ART AND PROBLEM THAT ARISES 
   Known devices for control of variable stator vanes in turbojets, particularly at the inlet of compressors, normally comprise a control device in the form of a ring surrounding the turbojet casing and several control levers. Each of these control levers has a first end installed on an outer pivot of a vane, for which the axis is the vane pivoting axis vane and a second end connected through an articulation to the control ring. The synchronised modification of the angular position of the vanes is therefore obtained by the ring rotating about the axis of the turbojet, so that it can follow the rotation movement of the control ring. The connection between each lever and the ring comprises at least one degree of freedom in rotation about an axis arranged approximately in the radial direction with respect to the ring. 
   The rotation movement applied to the vanes is essential to optimize the efficiency of the turbojet and the margin for compression. The precision and hysteresis are very important for modem high-pressure bodies in turbojets. In other words, the attachment of the end of the control lever to the outer pivot of the vane must be very precise. 
     FIG. 1  illustrates the assembly of the outer pivot  11  of a vane  10  on a bossing  21  of the casing  2  of a compressor stator. The outer pivot  11  is installed free to turn in a reaming  22  of the bossing  21  through two bushings  5  and  6  mounted tight or free to slide in the reaming  22 . Thus, a control lever  3  fixed on the end of the outer pivot  11  by means of a nut  4  provides a means of orienting the pitch of the vane  10 . 
   It is desirable to be able to reduce the contact pressure between the outer pivot  11  of the vane  10  and the bushings  5  and  6 , knowing that a space  12  remains between the two bushings  5  and  6 . However, an increase in the guidance height of the outer pivot  11  in the reaming  22  of the bossing  21  would increase the height of the bossing. Such an increase of the height of the bossing  22  obviously requires an increase in the outside diameter of the casing of the stator  2 , and therefore a greater increase in the cost of the as-cast casing and the machining to be done on it afterwards. 
   Therefore, the purpose of the invention is to increase the guidance height of the bushings  5  and  6  without increasing the outside diameter of the casing  2 , in order to keep the initial as-cast casing. 
   SUMMARY OF THE INVENTION 
   To achieve this, the main purpose of the invention is a guide system for the outer pivot of a variable stator vane for a turbojet stator with an outer pivot designed to pivot in a reaming with a determined length in the turbojet stator casing. 
   According to the invention, a single bushing is used made of a material with a low coefficient of friction, the length of which is greater than the length of the reaming, projecting outwards from the reaming, and for which the inside diameter is slightly greater than the outside diameter of the outer pivot, comprising a metallic liner for which the outside diameter is slightly greater than the inside diameter of the reaming and for which the inside diameter corresponds to the outside diameter of the bushing so that it is placed around the bushing, and it has an external hook at its outer end. 
   In the preferred embodiment of the invention, the bushing is metallic and a peripheral outer hook is provided at the outer end of the metallic liner, so as to be able to form a gripping element to extract the bushing. 
   It is also advantageous if the metallic liner is provided with a positioning edge to bear on the bushing of the stator casing. 

   
     LIST OF FIGURES 
     The invention and its various technical characteristics will be better understood after reading the following description with reference to the two figures: 
       FIG. 1  shows a sectional view of a guide assembly according to prior art, and 
       FIG. 2  shows a sectional view of the main embodiment of the guide system according to the invention. 
   

   DETAILED DESCRIPTION OF TWO EMBODIMENTS OF THE INVENTION 
     FIG. 2  shows the outer pivot  11  of a vane, or at least its end part and the bossing  21  of the stator casing of the turbojet. The end of the outer pivot  11  is provided with a thread  13  on which a nut  4  is screwed. This nut tightens the drive end of the lever  3  that will pivot the vane. 
   The outer pivot  11  is positioned in the reaming of the bossing  21  by means of a bushing  50  that extends over more than the length of the bossing  21  and for which the inside diameter is slightly greater than the outside diameter of the outer pivot  11 . 
   The bushing  50  is made of a composite material and it is always in direct contact through its inside diameter with the outside diameter of the outer pivot  11 . Due to the composite material from which it is made, the coefficient of friction in contact with another metallic part is very low, facilitating relative displacement between these two parts. A metallic liner  40  is inserted between the bushing  50  and the bossing  21  of the stator. This metallic liner  40  has an outside diameter slightly greater than the inside diameter of the reaming of the bossing  21  to enable attachment by banding. Moreover, it is provided with an outer hook  41  at its outer end  44  that is completed by a positioning edge  42  bearing on the bossing  21  of the stator. Relative sliding takes place firstly between the bushing  50  made of a composite material and the outer pivot  11  of the vane, and secondly between the bushing  50  made of a composite material and the metallic liner  44 . This system enables extraction of the bushing  50  made of a composite material and the metallic liner  44 , without disassembling the vane, by means of a claw  60  that is engaged under the hook  41 . Furthermore, if the bushing  50  made of a composite material degrades, the bossing  21  of the casing is protected by the metallic liner. 
   Note that the bushing  50  has an outer hook  51  that the bushing used to entrain it during its extraction. This outer hook  51  bears on the outer hook  41  of the metallic liner  40 . 
   Thus, it can be understood that the length of the contact surface of the outer pivot  11  is prolonged beyond the bossing  21 . 
   The bushing  50  can be extracted to replace it without needing to open the turbojet stator casing. 
   Moreover, this type of assembly provides better guidance of the outer pivot  11  without increasing the casing diameter. 
   In this first embodiment, the bushing is metallic, and in particular it is composed of a steel with good friction characteristics with the material used for the pivot, for example a bushing made of Z12CNDV12 with a pivot made of Z6NCT25.