Abstract:
One or more support elements radially extend through one or more openings defined in a turbine engine casing and are configured to centralize and at least partially support a rotor assembly of the engine during an engine disassembly or assembly procedure. The support elements are configured to transfer any rotor assembly weight loads to an engine casing while a bearing support of the rotor assembly is absent or removed.

Description:
TECHNICAL FIELD 
     The described subject matter relates generally to gas turbine engines and more particularly, to rotor centralization in gas turbine engine assembly. 
     BACKGROUND OF THE ART 
     A gas turbine engine generally includes one or more rotors supported by bearing structures in the engine. During an engine maintenance operation, such as on-wing hot section inspections of an aircraft turbine engine, in some engines an aft engine portion including an aft shaft bearing support structure is removed to provide access to the interior of the engine. Once the bearing support is removed, however, the rotor which the bearing supports tends to droop down, under its own weight, at the unsupported side to thereby create misalignment relative to the remaining support structures within the engine. This misalignment may cause damage to the rotor components at tight fit locations and may also cause difficulties during re-assembly of the engine. 
     Accordingly, there is a need to provide an improved approach. 
     SUMMARY 
     In accordance with one aspect, the described subject matter provides a method for supporting a gas turbine rotor assembly during engine assembly/disassembly, the rotor assembly having a central shaft extending substantially horizontally and supported by at least one bearing support structure, the method comprising: a) extending at least one elongate support element radially through an opening defined in a casing of the engine, the casing surrounding the rotor assembly; b) contacting the at least one support element with a portion of the rotor assembly at a location spaced apart from the at least one bearing support structure; and c) rigidly connecting the at least one support element to the casing of the engine, wherein the at least one support element is configured and positioned such that the at least one support element and the at least one bearing support structure cooperate to centralize and at least partially support the weight of the rotor assembly when a second bearing support structure of the engine is absent. 
     In accordance with another aspect, the described subject matter provides an apparatus for supporting a rotor assembly in a substantially centered position in a gas turbine engine, the apparatus comprising: at least three elongate support elements each extending radially through an opening defined in an exterior of a casing surrounding the rotor assembly, an inner end of each support element contacting with a periphery of the rotor assembly, the support elements each being connected at an outer end thereof to said exterior of the casing, the at least three support elements thereby configured to centralize and support the rotor assembly when a bearing support structure of the rotor assembly is absent. 
     In accordance with a further aspect, the described subject matter provides a method for supporting first and second rotor assemblies during engine assembly/disassembly, the second rotor assembly having a hollow central second shaft extending substantially horizontally and supported by at least at one bearing support structure, a first central shaft of the first rotor assembly co-axially extending through the hollow central second shaft of the second rotor assembly, opposite front and aft end portions of each of the first and second shafts being supported by a respective front support structure and an aft support structure within the engine during engine operation, the method comprising: a) radially extending three rigid elongate support elements through respective openings circumferentially spaced apart in a casing surrounding the first and second rotor assemblies, to lock the second rotor assembly in a substantially horizontal and centered position in the engine; and b) inserting at least one spacer in an annulus between the first and second shafts to support the first rotor assembly on the second rotor assembly, thereby locking the first and second shafts in the coaxial relationship. 
     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 drawings depicting aspects of the present invention, in which: 
         FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine according to one embodiment of the present description; 
         FIG. 2  is a simplified schematic transverse cross-section of the turbofan gas turbine engine taken along line  2 - 2  of  FIG. 1 ; 
         FIG. 3  illustrates an enlarged area of the engine, as circled and indicated by numeral  3  in  FIG. 1 , showing the adjustable connection of the bolt to the casing; and 
         FIG. 4  illustrates an enlarged area of the engine, as circled and indicated by numeral  4  in  FIG. 1 , showing a spacer placed between the coaxial high and low pressure spool shafts when a bearing structure is removed, according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a turbofan gas turbine engine which is taken as an exemplary application of the described subject matter, includes a fan case or engine nacelle  10 , a core casing  13 , a low pressure spool assembly  15  which includes a fan assembly  14  and a low pressure turbine assembly  18  connected by a central shaft  12 , and a high pressure spool assembly  23  which includes a high pressure compressor assembly  22  and a high pressure turbine assembly  24  connected by a central shaft  20 . The core casing  13  surrounds the low and high pressure spool assemblies  15 ,  23  defining a main fluid path (gas path) therethrough (not numbered). In the main fluid path there is provided a combustor  26  to generate combustion gases in order to power the high and low pressure turbine assemblies  24 ,  18 . 
     The shaft  20  of the high pressure spool assembly  23  is hollow to allow the shaft  12  of the low pressure spool assembly  15  to extend therethrough such that the shafts  12  and  20  and thus the low pressure and high pressure spool assemblies are disposed substantially coaxially within the engine. The common rotation axis of the shafts  12  and  20  defines the main central axis  30  of the engine. 
     A bearing support structure  16  which may be part of an intermediate case (not indicated) of the engine, supports the respective shafts  12  and  20  at a front or upstream portion thereof and a bearing support structure  32  which may be part of a mid turbine frame  28  positioned between the high pressure turbine assembly  24  and the low pressure turbine assembly  18 , supports the respective shafts  20 ,  12  at an aft or downstream portion thereof. Therefore, the bearing support structures  16  and  32  assure the centered position of both shafts  12  and  20 , and thus the low and high pressure spool assemblies within the engine. 
     Referring to  FIGS. 1-4 , one embodiment is shown for a method and apparatus for temporarily supporting or locking a gas turbine rotor assembly such that the respective shafts  12  and  20  are in their substantially centered positions within the engine during engine assembly/disassembly when the engine is disposed substantially horizontally and one of the bearing support structures  16 ,  32  is absent. During an engine maintenance operation such as in a hot section inspection, an aft portion of the engine including the mid turbine frame  28  is removed to provide rear end access to the interior of the engine. The shafts  12  and  20 , particularly the aft portion thereof, tends to drop down, causing deviation from the main central axis  30  of the engine, which is not desirable and should be avoided. Therefore, an apparatus for temporarily supporting or locking the shafts  12  and  20  in their substantially centered position within the engine is desired. 
     The apparatus according to this embodiment includes three rigid elongate support elements such as metal bolts  34  which radially extend through respective openings  36  circumferentially spaced apart in the core casing  13 , to lock the high pressure spool assembly  23  in a substantially horizontal and centered position in the engine. At this moment, the substantially horizontal and centered position of the shafts  12 ,  20  and thus the low and high pressure spool assemblies  15 ,  23  are assured by the bearing support structures  16  and  32 . Each of the bolts  34  is releasably secured to an outside of the core casing  13 . For example, a nut  35  is welded to the outside of the core casing  13 , aligning with each of the openings  36  for engagement with a threaded section  38  of each bolt  34 . 
     The threading engagement of the threaded section  38  of the bolt  34  with the nut  35  also functions as a means for adjusting the radial position of each bolt  34  relative to the core casing  13  in order to ensure a firm contact between an inner end of the bolt  34  and the high pressure spool assembly  23  which is represented by the high pressure compressor assembly  22  in  FIG. 2 . The threaded section  38  located in an outer end portion of each bolt  34 , may have a diametric dimension larger than the diameter of the remaining section of the bolt in order to allow the bolt  34  to conveniently extend through the opening  36  in the core casing  13 . 
     The three bolts  34  may be disposed in a same axial location of the engine and may in combination define a plane as shown in  FIG. 2 , substantially perpendicular to the central axis  30  of the engine. Alternatively, the three bolts  34  may be disposed in different axial locations of the engine. The axial location of the bolts  34  may be selected differently for different types of engines. For example, in the embodiment shown in  FIG. 1 , the bolts  34  are positioned in an axial location such that the inner ends of the bolts  34  are in contact with the high pressure compressor assembly  22 , such as in contact with a compressor platform (not numbered) thereof. The compressor platforms in combination define an inner surface of the main fluid path of the rotor assembly. 
     The one or more openings  36  in the core casing  13  which receive the respective bolts  34  to radially extend therethrough, may be existing ports defined in the core casing  13  such as a borescope port, if one or more such existing ports are available at a desirable axial location(s) of core casing  13 . Otherwise, openings  36  dedicated for temporarily receiving the respective bolts  34  are provided in the selected locations of the core casing  13  and are sealingly covered during engine operation. 
     The axial location of the bolts  34  should also be convenient for access from the outside of the core casing  13  to place and remove the bolts  34 . In the embodiment illustrated in  FIG. 1 , an aft section of the engine nacelle  10  is either openable or removable from the remaining section of the nacelle  10  which is mounted to the wing of an aircraft. 
     It should be noted that one of the bolts  34  as shown in  FIG. 2 , is disposed substantially in a vertical direction, which may not be necessary. However, if one of the bolts  34  is vertically disposed under the high pressure compressor assembly  22  as in an inverted image of  FIG. 2 , the bolt  34  in combination with the bearing support structure  16  will fully support the shaft  20  and thus the high pressure spool assembly  23  in its substantially horizontal and centered position within the engine when the bearing support structure  32  is removed. Therefore, the other two bolts  34  may not be in use. This single bolt support arrangement may be desirable in some circumstances according to various engine structures and/or tasks. 
     A spacer, for example a sleeve  40  according to this embodiment, may be provided to be inserted in an annulus  42  between the coaxial shafts  20  and  12  to support the inner shaft  12  on the outer shaft  20  in their coaxial relationship, thereby maintaining the low pressure spool assembly  15  in the substantially horizontal and centered position within the engine when the bearing support structure  32  is removed. The spacer  40  may have a small section (not numbered) having a thickness to allow easy insertion of the small section into the annulus  42  while substantially maintaining the coaxial relationship between the shafts  12  and  20 . The sleeve  40  may further include a large section (not numbered) having a diametric dimension larger than the diameter of the hole of the shaft  20 , to prevent over-insertion of the sleeve  40  from the aft end portion of the shaft  20  and to facilitate easy removal of the sleeve  40 . 
     It should be noted that placement of the bolts  34  and the sleeve  40  for temporarily locking the substantially horizontal and centered position of the respective shafts  20  and  12  within the engine during an engine maintenance operation, should be completed before removing the bearing support structure  32  and the bolts  34  and the sleeve  40  should be maintained in position until the maintenance operation is completed and the bearing support structure  32  is placed back in position. 
     In circumstances wherein the bearing support structure  16  which is located at the front portion of the high and low pressure spool assemblies  23 ,  16  is to be removed, the sleeve  40  should be placed in the annulus  42  between the inner and outer shafts  12 ,  20  at a front end portion of the shafts  12 ,  20 . If use of a sleeve  40  is not applicable due to engine structure, individual spacers such as three spacer blocks (not shown) instead of the sleeve  40  may be used. 
     The method and apparatus described above for temporarily locking and/or supporting the rotors of a gas turbine engine in their substantially centered position within the engine are not limited to use in an engine maintenance operation. The described method and apparatus may also be used for engine production assembly. The described method and apparatus may allow an engine assembly procedure in a more “ergonomic friendly position” with regard to assembly steps conducted and assembly platforms used in a horizontal engine assembly procedure with respect to those in an vertical engine assembly procedure. 
     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 departure from the scope of the described subject matter. For example, although a turbofan gas turbine engine having coaxially positioned high and low pressure spool assemblies has been used as an exemplary application of the described method and apparatus, the method and apparatus may be applicable to various types of gas turbine engines. The bearing support structures may not necessarily be associated with a mid turbine frame or an intermediate case but could be applied to any support structures depending on the particular engine structure for which the described method and apparatus are used. The use of elongate support elements such as the metal bolts and the use of spacers such as the sleeves, may not necessarily be combined and can be applicable one without the other depending on the particular engine structure in which they are used. Although a horizontal arrangement is discussed, the approach may likewise be applied to a gas turbine engine vertically-oriented during assembly or maintenance. 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.