Abstract:
A gas turbine having an annular casing with a series of circumferentially spaced openings defined therethrough; a plurality of vanes extending radially inwardly though respective casing openings, an outer end of the vanes projecting radially outwardly from the casing through the respective openings and located within the respective openings by grommets, and an inner end of the vanes being mounted to an inner portion of the casing; a flexible, segmented strap extending around the annular casing, surrounding the projecting outer ends of the vanes; and a spring radially loading the flexible strap configured to apply a tension force to the flexible strap.

Description:
TECHNICAL FIELD 
     The described subject matter relates generally to gas turbine engines, and more particularly to a stator vane restraining apparatus provided therein. 
     BACKGROUND OF THE ART 
     Gas turbine engine vane assemblies, such as those provided downstream of the engine fan, may have slots defined through the outer engine case for receiving and retaining the outer ends of the vanes in place. A grommet may be inserted in each of the slots to surround and isolate the respective vane from the shroud. However, during a foreign object damage event, a damaged vane can cut the grommet and cause damage to other surrounding components. An adhesive such as a potting compound is sometimes used, either in conjunction with or as a replacement for the grommet, but the use of such an adhesive generally complicates the installation and replacement of vanes. 
     Retaining straps have been developed but they are bulky and are not conducive to a confined space. 
     Accordingly, there is a need to provide an improved stator vane restraining apparatus for gas turbine engines, particularly when faced with a confined space. 
     SUMMARY 
     In one aspect there is provided a gas turbine engine having an annular casing with a series of circumferentially spaced openings defined therethrough; a plurality of vanes extending radially inwardly through respective casing openings, an outer end of the vanes projecting radially outwardly from the casing through the respective openings and located within the respective openings by grommets, and an inner end of the vanes being mounted to an inner portion of the casing; a flexible, segmented strap extending around the annular casing surrounding the projecting outer ends of the vanes; and a spring radially loading the flexible strap configured to apply a tension force to the flexible strap. 
     In another aspect there is provided a gas turbine engine having an annular casing with a series of circumferentially spaced openings defined therethrough; a plurality of vanes extending radially inwardly through respective casing openings, an outer end of the vanes projecting radially outwardly from the casing through the respective openings and located within the respective openings by grommets, and an inner end of the vanes being mounted to an inner portion of the casing; a flexible strap extending around the annular casing surrounding the projecting outer ends of the vanes; the improvement comprising a spring strip including an elongated web in the form of a circumferential ring and a plurality of laterally extending spring fingers projecting over the web in an alternating fashion from either edge of the web radially loading the flexible strap configured to apply a tension force to the flexible strap. 
     In a further aspect there is provided a vane retention apparatus for retaining an array of radially extending stator vanes arranged and protruding from an annular casing in a gas turbine engine; comprising a flexible, segmented strap extending around the annular casing surrounding the vanes; and a spring radially loading the flexible strap configured to apply a tension force to the flexible strap. 
     In a still further aspect there is provided a vane retention apparatus for retaining an array of radially extending stator vanes arranged and protruding from an annular casing in a gas turbine engine; comprising a flexible strap extending around the annular casing surrounding the vanes; the improvement comprising a spring strip including an elongated web in the form of a circumferential ring and a plurality of laterally extending spring fingers projecting over the web in an alternating fashion from either edge of the web radially loading the flexible strap configured to apply a tension force to the flexible strap. 
     In a still further aspect there is provided a method of retaining vanes in a gas turbine engine case, the case having a plurality of slots circumferentially distributed therearound, the vanes extending radially through each slot such that an end of the vane projects outwardly from the slot, the method comprising steps of placing a flexible strap made of a plurality of strap segments around the case to thereby surround the vane ends; joining the ends of the strap segments to provide a continuous strap and placing a circumferentially continuous spring strip between the strap and the case to apply a tension force on the flexible strap to tension the strap causing the spring strip and the strap to radially inwardly press the respective vanes in position. 
     Further details of these and other aspects of the described subject matter will be apparent from the detailed description and drawings included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG. 1  is a schematic cross-sectional view of a gas turbine engine illustrating the multishaft configuration; 
         FIG. 2 a    is a fragmented perspective view showing a detail of a first embodiment; 
         FIG. 2 b    is a fragmented side elevation of the detail shown in  FIG. 2   a;    
         FIG. 3 a    is a fragmented perspective view of a detail of another embodiment; 
         FIG. 3 b    is an expanded, fragmented, perspective view of the detail shown in  FIG. 3   a;    
         FIG. 4  is an enlarged, fragmented, perspective view of the detail shown in  FIGS. 3 a   ,  3   b;    
         FIG. 5  is a fragmented perspective view of another embodiment of the detail shown in  FIGS. 3 a   ,  3   b;    
         FIG. 6  is an enlarged, fragmented perspective view of a detail of the embodiment shown in  FIGS. 2 a , 2 b   ; and 
         FIG. 7  is a fragmented side elevation of another embodiment of the detail shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a gas turbine engine  10  which is taken as an exemplary application of the described subject matter. The gas turbine engine  10  generally comprises in serial flow communication, a fan  12  through which ambient air is propelled, a compressor section  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. Rotors of the respective fan  12 , compressor section  14  and turbine section  18 , rotate about an engine axis  11 . 
     Referring to  FIGS. 2 a  and 2 b   , a rotor assembly, which can be, for example, the fan  12  or a low pressure compressor of the compressor section  14  (both shown in  FIG. 1 ), includes rotor blades  22 , which are surrounded by an engine casing  24 . The casing  24  includes a tubular wall portion  24   a  extending downstream of the blades  22  to form part of a vane assembly  20 . The vane assembly  20  comprises an inner shroud  26  concentric with the casing  24  and located upstream of the rotating blades  22 . The inner shroud  26  and casing  24 , in combination, define the annular gas flow path  28  there-between. A plurality of vanes  30  extend radially between the outer casing  24  and the inner shroud  26 , upstream of the rotor blades  22 . Each of the vanes  30  has a radial, outer end portion forming a vane root  32  retained in the wall  24   a  of casing  24 , a radial inner end forming a vane tip  34  retained in the inner shroud  26 , and an airfoil portion  36  extending therebetween. The airfoil portion  36  of each vane  30  defines a relatively sharp leading edge  38  and a relatively sharp trailing edge  40 , such that an airflow passing through the vane assembly  20  will flow over the vane airfoil  36  from the leading edge  38  to the trailing edge  40 . 
     Throughout this description, the axial, radial and circumferential directions are defined respectively with respect to the central axis  11 , radius and circumference of the engine  10 . 
     As shown in  FIGS. 3 a , 3 b   , the outer casing  24  has a series of circumferentially spaced openings  46  defined, for example through the wall portion  24   a  of casing  24 . Each of the openings  46  has a profile similar to but slightly larger than the vane root  32  such that the vane root  32  is loosely received in the opening  46  and radially and outwardly projects from the outer surface of the outer casing  24 . 
     The vane root  32  includes an end platform  48  having a dimension greater than a dimension of the corresponding opening  46  defined in the casing  24 . A plurality of grommets  50  may be provided according to one embodiment, each grommet  50  sealing a gap between the outer platform portion  48  of one vane  30  and a corresponding opening  46 . The grommet  50  may be for example, an oblong elastic ring having an L-shaped cross-section with one leg inserted into the gap between the vane root  32  and a periphery of the opening  46  in the outer casing  24 , and with the other leg placed between an outer surface of the outer casing  24  and an inner surface of the end platform  48  of the vane  30 . 
     Referring to  FIGS. 3 a , 3 b    and  4 , there is shown a segmented flexible ring or strap  52 . The strap  52  is made up of segments  52   a ,  52   b  . . .  52   n . The end of each segment  52   a ,  52   b  or  52   n  includes a bead  54   a ,  54   b  respectively. Each bead  54   a  and  54   b , as shown in  FIG. 4 , includes an eyelet  56 . The beads  54   a  and  54   b  each have a reverse-angled notch  55  to accommodate a clasp  58 . In order to lock the clasp  58  in place, spring-type, C-shaped wires  60  are provided, as shown in  FIGS. 3 a , 3 b   . The reverse ends of the wires  60  engage eyelets  56  to lock the clasp  58  to the segmented strap  52 . 
     An alternative design is shown in  FIG. 5 . In this case, a pair of locking elements  62 , each having a pair of fingers  64 , perpendicular to the web of the element  62  adapted to be inserted laterally of the ends of the segmented strap  52   a  and  52   b  into the respective eyelets  56 . 
     As shown in  FIG. 6 , a spring strip  66  is wrapped around the vane assembly  20 , represented by vane  30 . The spring strip  66  is compressed and a flexible strap  52  is installed around the spring strip  66 . When tooling (not shown) is removed, the spring strip  66  presses against the strap  52  and each vane  30 , thus retaining the vane  30  with the required force. The spring strip  66  is made up of a continuous web  68  of spring steel, for instance, with alternating sprung fingers  70  and  72  extending laterally, each at an acute angle over the web  68 . It is noted that, at the root of each spring finger  70 ,  72 , where the stress is highest, if a crack forms and propagates, it will only affect one finger and the overall integrity of the spring strip  66  will not be significantly impacted. 
     Alternately, the spring strip  166  could be as shown in  FIG. 7  where the web  168  is spaced from the vane but the fingers  170  and  172  are in contact with the platforms of vanes  30 . 
     In either embodiment the spring  66 ,  166  is retained by ridges  74   a ,  74   b  on the vane head  48  where the strap  52  is retained by the ridge  74   b  on the aft side, and a support on the splitter  76  forward thereof. Also in both of these embodiments there are two optimally spaced points of contact both on the vane assembly  20  and on the strap  52 . This design feature is used to control the force distribution on the vane assembly  20  and the strap  52 . 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the described subject matter. For example, the fingers  170  and  172  in  FIG. 7  may be replaced by two continuous webs integrated with the continuous web  68  of spring steel. The two continuous webs each have a wedge smaller than ½ of the width of the continuous web  68  of spring steel and provide a radial spring function similar to that of the fingers  170  and  172 . Still other modifications which fall within the scope of the described subject matter will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.