Abstract:
A method for assembling a gas turbine engine that includes providing a low-pressure turbine inner rotor that includes a first plurality of turbine blade rows configured to rotate in a first direction, providing a low-pressure turbine outer rotor that includes a second plurality of turbine blade rows configured to rotate in a second direction that is opposite the first direction, coupling a turbine mid-frame assembly including a plurality of spokes within the engine such that the spokes are spaced axially forward of the inner rotor, coupling a bearing between the turbine mid-frame assembly and the inner rotor such that the inner rotor is rotatably coupled to the turbine mid-frame, and adjusting the plurality of spokes to align the bearing in a radial direction.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to aircraft gas turbine engines, and more specifically to a gas turbine engine and a method of assembling same.  
         [0002]     At least one known gas turbine engine includes, in serial flow arrangement, a forward fan assembly, an aft fan assembly, a high-pressure compressor for compressing air flowing through the engine, a combustor for mixing fuel with the compressed air such that the mixture may be ignited, and a high-pressure turbine. The high-pressure compressor, combustor and high-pressure turbine are sometimes collectively referred to as the core engine. In operation, the core engine generates combustion gases which are discharged downstream to a counter-rotating low-pressure turbine that extracts energy therefrom for powering the forward and aft fan assemblies. Within at least some known gas turbine engines, at least one turbine rotates in an opposite direction than the other rotating components within the engine  
         [0003]     At least one known counter-rotating low-pressure turbine has an inlet radius that is larger than a radius of the high-pressure turbine discharge. The increased inlet radius accommodates additional rotor stages within the low-pressure turbine. Specifically, at least one known counter-rotating low-pressure turbine includes an outer rotor having a first quantity of stages that are rotatably coupled to the forward fan assembly, and an inner rotor having an equal number of stages that is rotatably coupled to the aft fan assembly.  
         [0004]     During engine assembly, such known gas turbine engines are assembled such that the outer rotor is cantilevered from the turbine rear-frame. More specifically, the first quantity of rows stages are each coupled together and to the rotating casing to form the outer rotor. The outer rotor is then coupled to the turbine rear-frame using only the last stage of the outer rotor, such that only the last stage of the outer rotor supports the combined weight of the outer rotor and the rotating casing. The inner rotor is coupled to a shaft to facilitate driving at least one fan assembly. Moreover, the inner rotor is rotatably coupled to a turbine mid-frame using at least one bearing. Accordingly, the bearing must be properly aligned with respect to the turbine mid-frame to properly position the inner rotor within the gas turbine. However, properly positioning the bearing within the gas turbine engine results in an increased time required to assemble the gas turbine engine. Moreover, during engine operation, thermal expansion of the engine may result in a misalignment of the bearing with respect to the gas turbine engine outer casing.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005]     In one aspect, a method for assembling a gas turbine engine is provided. The method includes providing a low-pressure turbine inner rotor that includes a first plurality of turbine blade rows configured to rotate in a first direction, providing a low-pressure turbine outer rotor that includes a second plurality of turbine blade rows configured to rotate in a second direction that is opposite the first direction, coupling a turbine mid-frame assembly including a plurality of spokes within the engine such that the spokes are spaced axially forward of the inner rotor, coupling a bearing between the turbine mid-frame assembly and the inner rotor such that the inner rotor is rotatably coupled to the turbine mid-frame, and adjusting the plurality of spokes to align the bearing in a radial direction.  
         [0006]     In another aspect, a low-pressure turbine is provided. The low-pressure turbine includes an inner rotor including a first plurality of turbine blade rows configured to rotate in a first direction, an outer rotor including a second plurality of turbine blade rows configured to rotate in a second direction that is opposite the first direction, a turbine mid-frame assembly including a plurality of spokes, and a bearing coupled to the turbine mid-frame assembly and the inner rotor, wherein the spokes are adjustable to align the bearing in a radial direction.  
         [0007]     In a further aspect, a gas turbine engine is provided. The gas turbine engine includes an inner rotor including a first plurality of turbine blade rows configured to rotate in a first direction, an outer rotor including a second plurality of turbine blade rows configured to rotate in a second direction that is opposite the first direction, a turbine mid-frame assembly including a plurality of spokes, and a bearing coupled to the turbine mid-frame assembly and the inner rotor, wherein the spokes are adjustable to align the bearing in a radial direction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a cross-sectional view of a portion of an exemplary gas turbine engine;  
         [0009]      FIG. 2  is a cross-sectional view of a portion of gas turbine engine  10  shown in  FIG. 1 ; and  
         [0010]      FIG. 3  is an end view of the gas turbine engine shown in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]      FIG. 1  is a cross-sectional view of an exemplary gas turbine engine  10  that includes a forward fan assembly  12  and an aft fan assembly  14  disposed about a longitudinal centerline axis  16 . The terms “forward fan” and “aft fan” are used herein to indicate that one of the fans  12  is coupled axially upstream from the other fan  14 . In one embodiment, fan assemblies  12  and  14  are positioned at a forward end of gas turbine engine  10  as illustrated. In an alternative embodiment, fan assemblies  12  and  14  are positioned at an aft end of gas turbine engine  10 . Fan assemblies  12  and  14  each include a plurality of rows of fan blades  19  positioned within a nacelle  18 . Blades  19  are joined to respective rotor disks  21  that are rotatably coupled through a respective fan shaft  20  to forward fan assembly  12  and through a fan shaft  22  to aft fan assembly  14 .  
         [0012]     Gas turbine engine  10  also includes a core engine  24  that is downstream from fan assemblies  12  and  14 . Core engine  24  includes a high-pressure compressor (HPC)  26 , a combustor  28 , and a high-pressure turbine (HPT)  30  that is coupled to HPC  26  via a core rotor or shaft  32 . In operation, core engine  24  generates combustion gases that are channeled downstream to a counter-rotating low-pressure turbine  34  which extracts energy from the gases for powering fan assemblies  12  and  14  through their respective fan shafts  20  and  22 .  
         [0013]      FIG. 2  is a cross-sectional view of a portion of gas turbine engine  10  (shown in  FIG. 1 ).  FIG. 3  is an end view of gas turbine engine  10 . In the exemplary embodiment, low-pressure turbine  34  includes a radially outer rotor  110  that is positioned radially inwardly of outer casing  36 . Outer rotor  110  has a generally frusto-conical shape and includes a plurality of circumferentially-spaced rotor blades  112  that extend radially inwardly. Blades  112  are arranged in axially-spaced rows  114 . Although, the exemplary embodiment illustrates three rows  114  of blades  112 , it should be realized that outer rotor  110  may have any quantity of rows  114  of blades  112  without affecting the scope of the method and apparatus described herein. More specifically, outer rotor  110  includes M rows  114  of blades  112 .  
         [0014]     Low-pressure turbine  34  also includes a radially inner rotor  120  that is aligned substantially coaxially with respect to, and radially inward of, outer rotor  110 . Inner rotor  120  includes a plurality of circumferentially-spaced rotor blades  122  that extend radially outwardly and are arranged in axially-spaced rows  124 . Although, the exemplary embodiment illustrates only three rows  124  of blades  122 , it should be realized that inner rotor  120  may have any quantity of rows  124  of blades  122  without affecting the scope of the method and apparatus described herein. More specifically, inner rotor  120  includes N rows  124  of blades  122 . In the exemplary embodiment, M=N.  
         [0015]     In the exemplary embodiment, inner rotor blades  122  within rows  124  are axially-interdigitated with outer rotor blades  112  within rows  114  such that inner rotor rows  124  extend between respective outer rotor rows  114 . Blades  112  and  122  are therefore configured for counter-rotation of rotors  110  and  120 .  
         [0016]     In the exemplary embodiment, low-pressure turbine  34  also includes a rotor support assembly  130  that includes a stationary annular turbine rear-frame  132  that is aft of low-pressure turbine outer and inner blades  112  and  122 . A rotatable aft frame  134  is positioned aft of outer and inner blades  112  and  122 , and upstream from turbine rear-frame  132 . Frame  134  is coupled to an aft end of outer rotor  110  for rotation therewith and to facilitate providing additional rigidity for supporting blades  112 .  
         [0017]     Shaft  22  is rotatably coupled between inner rotor  120  and fan  14  such that inner rotor  120  is rotatably coupled to fan  14 . A first shaft bearing  140  is coupled to shaft  22  such that the weight of inner rotor  120  is distributed substantially equally about the circumference of gas turbine engine  10  via a spoked turbine mid-frame  150 , and such that high-pressure turbine is rotatably coupled to turbine mid-frame  150  through a bearing  142 . More specifically, gas turbine engine  10  includes a first housing  160  that is coupled to bearing  140  and a second housing  162  that is coupled to bearing  142 . Bearing  140  is positioned between high-pressure turbine  30  and shaft  22 . Housings  160  and  162  are coupled together to form a hub assembly  170 . In the exemplary embodiment, housings  160  and  162  are coupled together using a mechanical fastener  172 , such as a nut and bolt, for example. Accordingly, and in the exemplary embodiment, turbine mid-frame  150  facilitates supporting low-pressure turbine  34  and high-pressure turbine  30 .  
         [0018]     Turbine mid-frame  150  includes a plurality of yokes  180  that are coupled to hub  170 . Although only eight yokes  180  are shown, it should be realized that turbine mid-frame  150  may have any quantity of yokes  180  without affecting the scope of the methods and/or apparatus described herein. Each yoke  180  is substantially y-shaped and includes at least one opening  182  formed therein. In the exemplary embodiment, each yoke  180  includes a pair of openings  182  that are each selectively sized to receive an expandable pin  184  therein. Pins  184  are used to couple a spoke  186  to each respective yoke  180 . Accordingly, and in the exemplary embodiment, turbine mid-frame  150  includes eight spokes  186  that are each coupled to hub  170  using yokes  180  and pins  184 . More specifically, each respective spoke  186  includes a first end  190  that is coupled to a respective yoke  180  using pins  184 , and a second end  192  that extends through a respective opening  194  formed in outer casing  36 . Accordingly, in the exemplary embodiment, outer casing  36  includes eight openings  194  that are each sized to receive a respective spoke  186 . In the exemplary embodiment, each respective spoke second end  192  is threaded and selectively sized to receive a washer  196 , a first mechanical fastener  197 , and a second mechanical fastener  198 . In the exemplary embodiment, washer  196  is at least one of a belleville or a wave-type washer that is substantially cone-shaped, mechanical fastener  197  is a spanner nut, and mechanical fastener  198  is a lock nut.  
         [0019]     In the exemplary embodiment, during gas turbine engine  10  assembly, hub assembly  170  is coupled to spokes  186  using yokes  180  and pins  184 . Each respective mechanical fastener  197  is coupled to a respective spoke  186  such that washer  196  is at least partially compressed against casing  36 . More specifically, compressing each washer  196  against casing  36  induces tension into each respective spoke  186  to facilitate controlling the relative radial position of bearing  140 . Each respective spoke  186  is then retained in position as each fastener  198  is tightened against each respective fastener  197  such that fastener  197  is held in a relatively constant position with respect to each respective spoke  186 . In the exemplary embodiment, gas turbine engine  10  also includes a plurality of fairings  200 . More specifically, each respective fairing  200  is positioned around each respective spoke  186 , such that each fairing  200  facilitates channeling air around each respective spoke.  186 .  
         [0020]     During operation, radial forces generated during rotation of inner rotor  120  are transmitted to turbine mid-frame  150  via bearing  140 . More, specifically, as inner rotor  120  rotates, because each respective spoke  186  is in tension, turbine mid-frame  150  facilitates maintaining bearing  140  in a relatively constant axial and radial position with respect to casing  36 . Moreover, as a temperature of gas turbine engine  10  increases, washer  196  either expands or contracts to facilitate compensating for a thermal expansion of gas turbine engine  10 . More specifically, and in the exemplary embodiment, washer  196  functions as a spring to facilitate maintaining bearing  140  in a relatively constant axial and radial position when engine  10  is either expanding or contracting due to thermal expansion.  
         [0021]     Exemplary embodiments of a counter-rotating low-pressure turbines including an adjustable turbine mid-frame are described above in detail. The components are not limited to the specific embodiments described herein, but rather, components of each system may be utilized independently and separately from other components described herein. The adjustable turbine mid-frame described herein can also be used in combination with other known gas turbine engines.  
         [0022]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.