Abstract:
A rigid link of adjustable length includes a yoke with a transversal stud. The stud is mounted on the yoke with two bearings, each inside a through-orifice in the yoke. The axis of the stud is offset from the axis of the bearings whereby the link includes a stopping device in order to lock the stud in at least two different positions, relative to the yoke.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention concerns a mechanical linking means between a first member and a second member such as a control member and a controlled part. 
   2. Description of the Related Art 
   A gas turbine engine includes, for instance, doors blanking off the discharge apertures of the low pressure compressor in order to control the operation thereof. The doors are provided on the external wall of the tunnel downstream of the compressor, and distributed regularly around the axis of the engine. There may be ten, for example. They are mounted generally in order each to tip around an axis situated in a plane transversal to the axis of the engine. Simultaneous opening of the doors is actuated by means of an annular control member, brought into rotation by an appropriate jack. The rotational movement of this ring is transformed into an axial movement by square-shaped transmission parts. A branch thereof is connected to a yoke integral with the door that it controls by dint of a link. 
   Taking the assembly and manufacture tolerances of the parts into account, a means for adjusting the length of the link actuating each door is required. This means enables to ensure that all the doors occupy the same reference position. Notably, when the control ring is placed in the closing position of the doors, the latter should all be able to blank off the apertures correctly. 
   With reference to  FIG. 1 , in the configuration of the current state of the art, a link  1 ′ includes a fastening ring  30 ′ at one end and a yoke  20 ′ at the other end. It is composed of two sections: the ring is provided with a threaded rod  31 ′ co-operating with a tapered orifice  11 ′ in the yoke  20 ′. The length of the link  1 ′ is adjusted by rotating one part relative to the other and both parts are immobilised together by means of a nut  32 ′ locking a washer  33 ′ slid onto the threaded rod  31 ′. The nut  32 ′ is locked, for its own part, by a metal wire. The assembly and the adjustment of the link are not easy since one of the attachments must be released in order to proceed to the adjustment in length. The adjustment accuracy is determined; it is given by the variation in length generated by the rotation over half a turn. In this example, the adjustment is 0.45 mm by semi-turn. 
   This type of link exhibits the shortcoming in aeronautics that it must be made of a suitable material compatible with a threading. Steel is therefore used. However, the mass of this metal is high with respect to other metals used generally in this field, and especially it is corrosion sensitive. Besides, the link is composed of five different parts, which is a detrimental factor in terms of management and maintenance. 
   SUMMARY OF THE INVENTION 
   The purpose of the applicant is to provide a rigid link between a first member and a second member, including a means for adjusting the distance between both members, said link including a yoke with a stud co-operating with a fastening tab on the first member. 
   This target is met thanks to a link whereof the stud is mounted on the yoke by bearing-forming means inside a recess laid out on the yoke, whereas the axis of the stud and the axis of the bearing-forming means are not confused, whereby the link includes stopping means in order to lock the stud in at least two different positions, relative to the yoke. 
   The link of the invention advantageously dispense with threaded parts. Consequently, a metal lighter than steel may be used, an aluminium-based alloy for example. The latter, besides, resists to corrosion much better than steel. The production of the link is also simplified since it may be composed of a single piece, where the adjustment means are transferred to the assembly yoke. The structure of this link also enables an adjustment at least equal to the requirements set, in situ, without the adjuster having to dismantle the link at one of its ends. 
   This invention is described for a rigid transmission link between a ring controlling the flow discharge doors of a compressor. However, it may be extrapolated to any application implementing a rigid transmission link whereof the length may be adjusted at predetermined positions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages will appear when reading the following description of an embodiment of the invention, with reference to the appended drawings whereon:
           FIG. 1  shows a side view of a link of the previous art,     FIG. 2  shows a perspective view of the link of the invention,     FIG. 3  shows a side view of the link according to  FIG. 2 ,     FIG. 4  shows a sectional view of the link along the direction III-III of  FIG. 3 ,     FIG. 5A  shows a side view of the stud of the invention,     FIG. 5B  shows an axial view of the stud of  FIG. 5A ,     FIG. 5C  shows an axial view of the bearing-forming means associated with the stud of  FIG. 5B ,     FIGS. 6A and 6B  show, at another scale, embodiment variations of two heads enabling different adjustments.       

   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The link  1  of the invention includes a body with a stem-shaped element  10  extended on one side by a yoke-shaped attachment means  20  for assembly on a fastening tab integral with a first mechanical member, not represented. It may be for instance, a discharge door of a low pressure compressor in a gas turbine engine. At the other end, the stem is integral with a ring  30  fitted with a ball-joint  32  for assembly on a fastening tab integral with a second member to be connected to the former. The ball-joint  32  is drilled with a cylindrical aperture  33  of axis  33 A for accommodating a stud, not represented, for attaching the second member to said fastening tab. It may be the square or the bellcrank connected to the discharge door control ring of the compressor. 
   As can be seen on  FIG. 4 , the yoke  20  is composed of two fastening tabs  22  and  24  parallel relative to one another and each fitted with a circular through-orifice  23  and  25  of axes  23 A and  25 A. The axes  23 A,  25 A of both orifices  23 ,  25  are collinear. A stud  40  is mounted on the yoke between both tabs by bearing-forming means  42  and  44 . The stud  40 , as can be seen on  FIG. 5 , is cylindrical in shape with a circular section, of axis  40 A. A bearing-forming means  42  is here interconnected with the stud  40  and is integral part therewith. The second means  44  is mounted slidingly on the stud. The first means  42  includes a cylindrical bearing surface  421  with a circular section. Its axis  42 A is parallel to the axis of the stud  40 A. Both axes  40 A and  42 A are not collinear; they are apart from one another by a set distance ‘e’. The first means  42  also includes a portion forming a bearing head  422 . This head is also cylindrical with at least one plane surface  422 A forming a stop. This surface  422 A is parallel to the axis  42 A of the bearing surface. 
   The second bearing-forming means  44 , as can be seen on  FIG. 5C , is composed of a circular cylindrical portion  441  of axis  44 A confused with the bearing axis  42 A of the first means  42 . The cylindrical portion is provided with an orifice  443  centred on the axis of the stud  40 . The second means  44  also includes a head  442  with, like the first head, at least one plane stopping surface  442 A parallel to the axis of the stud. 
   The external surface of the branch  22  of the yoke  20  comprises a stopping means  22 B, here in the form of a plane surface. Its distance to the axis  23 A of the orifice  23  provided in the branch  22  is slightly greater than or equal to the distance between the stopping surface  442 A, on the head  442 , of the bearing-forming means  44 . 
   Similarly, the external surface of the branch  24  of the yoke  20  includes a stopping means  24 B. Its distance to the axis  25 A of the orifice  25  provided in the branch  24  is slightly greater than or equal to the distance between the stopping surface  422 A, on the head  422 , of the bearing-forming means  42 . 
   As can be seen on  FIG. 5C ; the surface of the cylindrical bearing  441 , seen from above, forms a circle like the cylinder of the stud. The centre of the cylinder  40  is offset relative to the centre of the bearing circle  441 . On  FIG. 4 , the surface  442 A abuts against the stopping surface  22 B. Similarly, the stopping surface  422 A abuts against the stopping surface  24 B on the other tab of the yoke. 
   The heads  422  and  442  have in this embodiment a pentagonal shape and include each five stopping surfaces  422 A to E and  442 A to E. 
   According to the arrangement of the stud relative to the yoke, the distance between the axis  40 A of the stud and the axis  33 A of the ball-joint is adjusted. In this embodiment, there are three adjustment lengths. It suffices to place the heads  442  and  422  in the appropriate position to present the suitable pair of stopping surfaces,  442 A- 422 A,  442 B- 422 B,  442 C- 422 C,  442 D- 422 D or  442 E- 422 E. 
   A link is used as follows in the example of control system for the discharge doors of a low pressure compressor of a gas turbine engine. The doors are in a position blanking off the apertures. For each door, a control square has been mounted on the casing so that said square may rotate around an axis with radial direction. One end of the square is integral with the ring controlling all the doors. The other end is connected by dint of the link to the corresponding door. 
   One starts by installing the link by the end including the ring, then one places the yoke at the other end, facing the fastening tab of the second member. The stud  40  is slid through the orifices  23  and  25  of the yoke  20  and rotated around its axis until it finds the appropriate stopping surface of its head  422 . The distance between the axes  33 A and  40 A corresponds to the reference distance between both members to be connected with an accuracy related to the eccentricity ‘e’. When the head  422  is in place, one inserts the second member  44  into the same position. 
   A stud has been described with an integral bearing means  42 . The invention also includes the variation not represented where said means is distinct, like the second bearing means  44 . 
   This first embodiment includes heads pentagonal in shape, enabling three adjustment positions of the distance between the axes  33 A and  40 A. It can be understood that one may choose other positions between the offset-centres or other polygonal shapes; in particular, the heads may be square or hexagonal in shape, for another number of adjustment positions. 
   This present invention advantageously enables to suit the adjustment accuracy to the application easily. The stud whereof the head has an appropriate number of adjustment positions is then selected. 
   On  FIG. 6A , for instance, the head of the bearings is square. The axis  60 A of the stud is offset relative to the axis of the bearing  62 A. The adjustments in this arrangement are three in number. In the example of  FIG. 6B , the head of the bearings is hexagonal. The axis of the stud  60 ′A is offset relative to the axis  62 ′A of the bearing, the adjustments are here four in number.