Abstract:
A device for centering a component positioned inside and rotationally secured at its end to a turbine hollow shaft positioned in a gas turbine engine. The device includes a sheath enclosing and solidly joined to the component, and an elastic ring interposed between the sheath and the shaft. The ring includes a plurality of pairs of opposed shoes extending radially outwardly therefrom and configured to radially expand so as to extend against an inner portion of the shaft. The shoes include thin, elastic cylindrical walls having an external diameter configured slightly less than the internal diameter of the shaft prior to assembly onto the shaft, and arranged to expand to the inner wall of the shaft when assembled thereon.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a device for centering a tube or a component within a hollow turbine shaft of a gas turbine engine, the tube or the component being rotationally ganged at its ends to the shaft, the device comprising a sheath rigidly affixed to and enclosing the tube or component, further comprising an elastic ring inserted between the sheath and the shaft and fitted with radially outward running shoes, and means allowing ring expansion subsequent to assembly in order that the shoes shall rest against the shaft&#39;s inside wall. 
   As regards to aircraft gas turbine engines, they comprise a tube coaxially configured inside the hollow central shaft to connect the low pressure turbine to the low pressure compressor and, as called for, to the fan, the tube allowing exposing the rotor-supporting, front and rear bearing casings to the ambient air. The tube also allows evacuating a given rate of oil bearing air. 
   This tube runs along the full length of the turbine shaft and at its ends is fitted with rest means rotationally connecting it to the shaft. This tube is quite elongated and exhibits a relative thin wall because it doesn&#39;t transmit any torque. Accordingly, its transverse moment of inertia is small. Consequently, if there were even very slight mass imbalances, they would degrade tube behavior during gas turbine engine operation and flexural excursions might arise in it with ensuing damage to it and adjacent components. 
   As a precaution, at least one centering component is mounted between the two end supports of this tube in order to maintain the central tube zone within the hollow shaft. 
     FIG. 1  shows an aircraft gas turbine engine  1  having an axis X and comprising a tube  3  with axis X inside the low pressure turbine shaft  2 . This tube  3  enables the rotor-supporting front and rear casings  4  and  5  of the front and rear bearings  6  and  7  to communicate with the ambient air. This tube  3  is rotationally ganged at its ends to the low pressure turbine shaft  2 . Two centering systems  8   a ,  8   b  are configured between the ends of the tube  3  to coaxially keep the segments of this tube  3  within the shaft  2  and in this manner to prevent it from flexing.  FIGS. 2 and 3  show the configuration of the present-day centering systems  8   a  and  8   b . The thin-walled tube  3  comprises a reinforced annular segment  3   a  around which is mounted a sheath  9  which is fitted at its end  9   a  with an outside thread  10  and at its other end  9   b  with a peripheral conical wall  11  diverging into the extension of the thread  10 . 
   An elastic split ring  12  of which the inside wall also is conical is moved by the threaded end  9   a  onto the sheath  9 . A nut  14  fitted with an inside thread matching the thread  10  displaces the ring  12  on the conical wall  11 . As a result the ring  12  expands. 
   The elastic split ring  12  is shown in detail in  FIG. 3  and comprises a peripheral surface of substantially square cross-section with rounded corners  15 , the curvature of the latter being matched to the particular inside diameter of the shaft  2 . When the ring  12  expands, the corners  15  come to rest against the inside wall of the shaft  2 . 
   In this manner the elastic ring  12  exhibits four cross-sectionally radial rigid shoes that are externally bound by the corners  15  and are pairwise connected by ring segments of lesser radial cross-section. 
   The elastic ring  12  comprises a lateral slit  16  and behaves like an assembly of two cantilevered beams embedded in the zone  17  which is diametrically opposite the slit  16 . Along the beams, the magnitudes of bending torque and of deflection are variable and, as a result, the support stresses are not the same at the four support zones. Hence, the contact stresses between the elastic ring  12  and the turbine shaft  2  also are different along the support sites. 
   Occasionally the magnitudes of the stresses are prohibitive and, as a result, imprints may be imparted to the inside bore of the turbine shaft  2 : serious consequences may ensue considering that this shaft  2  undergoes considerable torsion. 
   SUMMARY OF THE INVENTION 
   The objective of the present invention is to propose a device maintaining a tube inside a shaft in the manner already cited above and precluding imprinting the shaft&#39;s bore. 
   This goal is attained by the invention in that the shoes are fitted with elastic and thin, cylindrical walls, and at rest, exhibit an outside diameter which is slightly different from the inside hollow shaft&#39;s inside diameter while after assembly they will hug the shaft&#39;s inside wall. 
   Accordingly, the radial cross-section of the shoes of the invention, as seen in a plane containing the gas turbine engine&#39;s axis of rotation, is clearly less than that of the shoes of the prior art. This design ensures that the radial stresses caused by the tightening torque shall be spread over a larger area, and it reduces the contact stresses between the shoes and the turbine shaft&#39;s bore. 
   In a first embodiment of the present invention, the ring is an elastic, split ring and the means allowing ring expansion following assembly comprise a male/female cone system configured on the sheath and on the ring, with a nut assuring axial ring displacement by being tightened on the sheath, the ring&#39;s radial expansion, and the deformation of the thin shoes walls. 
   In a first variation of the first embodiment of the present invention, the split ring comprises a substantially cylindrical inside wall and each foot is connected by its center axial zone to the cylindrical inside wall by a radial wall and exhibits a flexible lip on each side of the radial wall. 
   In this first variation, the outside diameter of the shoes at rest is larger than the inside shaft diameter and will decrease during assembly. 
   In a second variation, the split ring comprises several cylindrical segments alternating with the shoes, the ends of each foot being respectively imbedded in the two adjacent cylindrical segments. 
   In this second embodiment, the outside diameter of the shoes at rest also is less than the inside shaft diameter and will increase during assembly. 
   In a second embodiment of the present invention, each foot comprises a thin cylindrical wall of which the outside diameter at rest is less than the inside shaft diameter, and each foot is fitted, at each thin wall end, with an arcuate element, the elements being axially kept in place by two elastic clips to constitute a ring which, at rest, exhibits a lesser diameter than the outside sheath diameter. 
   In a first variation of the second embodiment of the present invention, the sheath comprises a shoulder to axially maintain the elements on a cylindrical sheath segment, the sheath furthermore being fitted on the shoulder side with a bevel allowing the ring and the clips to expand during ring assembly by tightening a nut on the sheath. 
   Advantageously, four shoes regularly distributed around the gas turbine engine&#39;s axis of rotation are provided. 
   In another variation of the second embodiment of the present invention, the means allowing ring expansion during assembly comprise a male/female cone system configured on the sheath and on the elements, and a nut, which when tightened onto the sheath, assures the axial displacement of the elements, the radial expansion of these elements and of the clips, and the deformation of the thin shoes walls. 
   In another advantageous feature of the present invention, and for the case of rest, the circle subtended by the outer sides of the shoes shall be of a diameter slightly less than that of the shaft&#39;s in order to allow installing the device of the invention in the turbine bore before mounting the tube. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and features of the present invention are elucidated in the following illustrative description and in relation to the attached drawings. 
       FIG. 1  is a cross-section of a gas turbine engine showing the tube centering device mounted coaxially inside the low pressure turbine shaft, 
       FIG. 2  is a section of a centering device of the prior art, 
       FIG. 3  is a front view of the elastic split ring of the prior art, 
       FIG. 4  is a front view of the elastic split ring of the first variation of the first embodiment of the present invention, 
       FIG. 5  is a section along a radial plane through the gas turbine engine&#39;s axis of rotation of the centering device of the invention under a first variation of a first embodiment, the split ring being sectioned along line V—V of  FIG. 4 , 
       FIG. 6  shows the configuration of a foot of the first variation prior to assembly, 
       FIG. 7  shows the foot configuration following assembly, 
       FIG. 8  is a front view of an elastic split ring of a second variation of the first embodiment of the invention, 
       FIG. 9  is a section along an axial plane through the gas turbine engine&#39;s axis of rotation of the centering device of the invention according to the second embodiment variation, the split ring being sectioned along line IX—IX of  FIG. 8 , 
       FIG. 10  shows the outside radius of the foot of  FIG. 8  before and after assembly, 
       FIG. 11  is a front view of an elastic ring of a second embodiment of the present invention and comprising four independent shoes which are axially maintained in position by two elastic clips, 
       FIG. 12  is a section in an axial plane through the gas turbine engine&#39;s axis of rotation of the centering device of the second embodiment of the invention in the assembled state, the ring being sectioned along the line XII—XII of  FIG. 11 , 
       FIG. 13  is a detail of the variation of the outside foot radius while the ring of  FIG. 11  is expanding, and 
       FIG. 14  is similar to  FIG. 12  and shows an embodiment variation of the ring&#39;s expansion system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Having already been discussed in the above introduction, further discussion of  FIGS. 1 through 3  is not necessary. 
   The tube  3  of the invention comprises, as in the prior art, a reinforced front segment, omitted from  FIGS. 4 through 14 , around which is mounted a sheath  9  comprising a first threaded end  9   a  and a second threaded end  9   b  running in the extension of the threaded end  9   a  to receive an expansible ring  20 . The ring  20  is retained on the sheath  9  by a nut  14  fitted with an inside thread which cooperates with the thread of the end  9   a  of the sheath  9 . The ring  20  is inserted between the sheath  9  and the inside wall  21  of the turbine shaft  2 . For sake of clarify, only the inside wall  21  of the turbine shaft  2  is shown in  FIGS. 4 through 14 . After the centering device has been inserted into the bore of the turbine shaft  2  and after the tube  3  has been installed, the inside wall  21  exhibits a radius E/2 relative to the ring  20 . 
   In a first embodiment of the invention, the ring  20  is fitted with a slit  22  of the already described state of the art. 
   In a first variation of the first embodiment of the invention shown in  FIGS. 4 through 7 , the expansible ring  20  comprises an inner and substantially cylindrical wall  23  which, following assembly, will enclose the sheath  9 . 
   Several or four radial walls  24  bearing at their ends cylindrical and thin-walled shoes  25  run around the inside wall  23 , and radially outward in orthogonal planes intersecting each other along the gas turbine engine&#39;s axis of rotation X. 
   Each foot  25  is fitted on each side of the radial support wall  24  with a flexible lip  26   a  and  26   b  respectively. 
   When the shoes  25  are at rest, as shown in  FIG. 6 , the outside radius of their cylindrical wall is larger than the inside turbine shaft radius E/2. In this case, the diameter of the circle touching the four shoes is greater than the turbine shaft&#39;s inside diameter E. 
   During assembly, the ends of the lips  26   a  and  26   b  shall bend when coming into contact with the turbine shaft&#39;s bore as the nut  14  is tightened. In the assembled configuration shown in  FIG. 7 , the outside radius of the cylindrical wall of each foot  25  will equal the turbine shaft radius E/2. Accordingly, each foot  25  rests homogeneously over its entire outside surface against the inside wall  21 . 
   When the nut  14  is tightened, the ring  20  slides over the end  9   b  of the sheath  9  and expands radially outward due to a conical male/female system configured at the outer periphery of the sheath  9   b  and at the inner periphery of the ring  20 . 
   The ring  20  and the untightened nut  14  are placed on the sheath  9  when the above centering device is mounted in the turbine shaft. The assembly is inserted into the turbine shaft. The nut  14  is used to apply a tightening torque in order that the axial center zones of the shoes  25  are situated a distance E/2 from the axis X, whereby the lips  26   a  and  26   b  are made to bend. Next the tube  3  is mounted in place. 
   Regarding a second variation shown in  FIGS. 8 through 10  of the first embodiment of the invention, differing solely from the first variation by the geometry of the ring  20 , the four shoes  25  coming to rest against the turbine shaft&#39;s inside wall  21  consist of thin cylindrical walls exhibiting in their rest mode an outside radius R 2  less than the turbine shaft&#39;s inside radius E/2. 
   The radially inner part of the ring  20  comprises several cylindrical, circumferentially spaced cylindrical segments  27  alternating with the shoes  25 . 
   The ends  25   a  and  25   b  of each foot  25  are respectively imbedded in two adjacent segments  27 . A slit  22  is subtended in one of the segments  27 . 
   Expansion of this ring  20  on the sheath  9  by the cone system and by tightening the nut  14  entails an increase of the radius of the circle touching the four shoes  25  at rest. 
   This second embodiment variation of the centering device is mounted in the same manner as the above described first variation. The foot center zones  28  will bend as a tightening torque is applied by the nut  14  to increase the radius R 2  until the shoes&#39;s outside radius is the same as the radius E/2 of the turbine shaft. Next, the tube  3  is installed. 
   The areas of the shoes  25  resting against the turbine shaft&#39;s inside wall  21  are substantially larger than those of the state of the art discussed in the above introduction, where the shoes are undeforming rigid blocks alternating with cross-sectionally smaller connecting segments. 
   In a second illustrative embodiment of the present invention shown in  FIGS. 11 through 14 , the elastic ring  20  consists of four independent identical elements  30  joined by two annular elastic clips  31 . 
   In its center zone, each element  30  comprises a foot  25  constituted by a thin cylindrical wall of the outside radius R2 less than the turbine shaft&#39;s radius E/2. The ends  25   a  and  25   b  of each foot  25  are imbedded respectively in two small blocks  31   a  and  31   b  which constitute the radially inner and lateral parts of an element  30 . The small blocks  31   a ,  31   b  are arcuate. 
   In this manner, each element/sub-assembly  30  assumes an arcuate shape no wider than 90°. 
   The four elements  30  are joined to each other by two clips  31  received in grooves in the outer peripheral wall of the small blocks  31   a  and  31   b , one of the grooves being situated near the front face of the ring  20  and the other near its rear face. 
   Small gaps  32  separate the four elements  30  retained by the clips  31 . These small gaps  32  will widen when the ring  20  expands. 
     FIG. 12  shows an embodiment variation of the above assembly of the ring  20  to the sheath  9 . The sheath comprises a shoulder  33  running radially outward and supporting a front face of the ring  20 . The ring  20  covers a cylindrical sheath portion  34  of a diameter larger than the outside diameter of the threaded end  9   a  and larger than the inside diameter of the ring  20  before it is mounted on the sheath  9 . Near the side of the shoulder  33 , the small blocks  31   a  and  31   b  are fitted with a bevel  35  provided to expand the ring  20  and the clips  31  when the ring moves on the cylindrical part  34  consequent to tightening the nut  14 . 
   In this rest configuration, the circle touching the outside of the four shoes  25  subtends a radius which is less than the turbine shaft&#39;s inside radius E/2. Once the tube has been placed in the turbine shaft, the center axial zones  28  of the shoes  25  are bent and the outside radius of the shoes  25  is substantially equal to the turbine shaft&#39;s inside radius E/2. 
     FIG. 14  shows another embodiment variation of assembling the ring  20  to the sheath  9 . As in the first embodiment of the present invention, the sheath  9  and the small blocks  31   a  and  31   b  comprise a male/female cone system allowing the ring  20  and the clips  31  to expand when the nut  14  is being tightened. 
   Obviously, a washer  36  may be inserted between the ring  20  and the nut  14  in all the above discussed embodiments and their variations. 
   The above discussion relates to four shoes  25  on each expansible ring  20 . Obviously, the number of shoes may be varied, preferably however being even. What matters foremost is that the shoes be regularly distributed around the turbine&#39;s axis of rotation X. 
   Because the shoes  25  consist of thin and deforming cylindrical walls, the contacting areas of these shoes are relatively large and the stresses are spread over a large surface without entailing prohibitive contact stresses between the centering device and the turbine shaft&#39;s bore. As a result, the turbine shaft remains free of imprints. 
   Moreover, the second embodiment of the present invention provides improved stress distribution over all shoes due to its symmetry.