Abstract:
A method for assembling a gas turbine engine includes coupling a low-pressure turbine to a core turbine engine, coupling a gearbox to the low-pressure turbine, coupling a first fan assembly to the gearbox such that the first fan assembly rotates in a first direction, and coupling a mechanical fuse between the first fan assembly and the low-pressure turbine such that the mechanical fuse fails at a predetermined moment load.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to gas turbine engines, and more specifically to gas turbine engine assemblies and methods of assembling the same.  
         [0002]     At least some known gas turbine engines include a forward fan, a core engine, and a power turbine. The core engine includes at least one compressor, a combustor, a high-pressure turbine, and a low-pressure turbine coupled together in a serial flow relationship. More specifically, the compressor and high-pressure turbine are coupled through a shaft to define a high-pressure rotor assembly. Air entering the core engine is then mixed with fuel and ignited to form a high energy gas stream. The gas stream flows through the high-pressure turbine, rotatably driving it, such that the shaft that, in turn, rotatably drives the compressor.  
         [0003]     The gas stream expands as it flows through the low-pressure turbine. The low-pressure turbine rotatably drives the fan through a low-pressure shaft such that a low-pressure rotor assembly is defined by the fan, the low-pressure shaft, and the low-pressure turbine. To facilitate increasing engine efficiency, at least one known gas turbine engine includes a counter-rotating low-pressure turbine that is coupled to a counter-rotating fan and/or a counter-rotating booster compressor.  
         [0004]     To assemble a gas turbine engine including a counter-rotating low-pressure turbine, an outer rotating spool, a rotating frame, a mid-turbine frame, and two concentric shafts are installed within the gas turbine engine to facilitate supporting the counter-rotating turbine. The installation of the aforementioned components also enables a first fan assembly to be coupled to a first turbine and a second fan assembly to be coupled to a second turbine such that the first and second fan assemblies each rotate in the same rotational direction as the first and second turbines. Accordingly, the overall weight, design complexity, and/or manufacturing costs of such an engine are increased. Moreover, to facilitate supporting the fan assemblies, at least one of the fan assemblies is supported on a plurality of bearing assemblies. During operation of the engine, a fragment of a fan blade may become separated from the remainder of the blade. Accordingly, a substantial rotary unbalance load may be created within the damaged fan and carried substantially by the fan shaft bearings, the fan bearing supports, and the fan support frames.  
         [0005]     To minimize the effects of potentially damaging abnormal imbalance loads, known engines include support components for the fan rotor support system that are sized to provide additional strength for the fan support system. However, increasing the strength of the support components may also increase an overall weight of the engine and decrease an overall efficiency of the engine when the engine is operated without substantial rotor imbalances. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0006]     In one aspect, a method of assembling a turbine engine is provided. The method includes coupling a low-pressure turbine to a core turbine engine, coupling a gearbox to the low-pressure turbine, coupling a first fan assembly to the gearbox such that the first fan assembly rotates in a first direction, and coupling a mechanical fuse between the first fan assembly and the low-pressure turbine such that the mechanical fuse fails at a predetermined moment load.  
         [0007]     In another aspect, a counter-rotating fan assembly is provided. The counter-rotating fan assembly includes a gearbox coupled to a low-pressure turbine, a first fan assembly coupled to the gearbox, the first fan assembly comprising a disk and a plurality of rotor blades coupled to the disk and configured to rotate in a first rotational direction, and a mechanical fuse coupled between the first fan assembly and the low-pressure turbine such that the mechanical fuse fails at a predetermined moment load.  
         [0008]     In a further aspect, a turbine engine assembly is provided. The turbine engine assembly includes a core turbine engine, a low-pressure turbine coupled to the core turbine engine, a gearbox coupled to the low-pressure turbine, a first fan assembly coupled to the gearbox, the first fan assembly comprising a disk and a plurality of rotor blades coupled to the disk and configured to rotate in a first rotational direction, and a mechanical fuse coupled between the first fan assembly and the low-pressure turbine such that the mechanical fuse fails at a predetermined moment load.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a cross-sectional view of a portion of an exemplary turbine engine assembly;  
         [0010]      FIG. 2  is an enlarged cross-sectional view of a portion of the counter-rotating fan assembly shown in  FIG. 1 ; and  
         [0011]      FIG. 3  is an enlarged cross-sectional view of a portion of the counter-rotating fan assembly shown in  FIG. 2  that includes a mechanical fuse. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]      FIG. 1  is a cross-sectional view of a portion of an exemplary turbine engine assembly  10  having a longitudinal axis  11 . In the exemplary embodiment, turbine engine assembly  10  includes a core gas turbine engine  12 , a low-pressure turbine  14  that is coupled axially aft of core gas turbine engine  12 , and a counter-rotating fan assembly  16  that is coupled axially forward of core gas turbine engine  12 .  
         [0013]     Core gas turbine engine  12  includes an outer casing  20  that defines an annular core engine inlet  22 . Casing  20  surrounds a low-pressure booster compressor  24  to facilitate increasing the pressure of the incoming air to a first pressure level. In one embodiment, gas turbine engine  12  is a core CFM56 gas turbine engine available from General Electric Aircraft Engines, Cincinnati, Ohio.  
         [0014]     A high-pressure, multi-stage, axial-flow compressor  26  receives pressurized air from booster compressor  24  and further increases the pressure of the air to a second, higher pressure level. The high-pressure air is channeled to a combustor  28  and is mixed with fuel. The fuel-air mixture is ignited to raise the temperature and energy level of the pressurized air. The high energy combustion products flow to a first or high-pressure turbine  30  for driving compressor  26  through a first drive shaft  32 , and then to second or low-pressure turbine  14  to facilitate driving counter-rotating fan assembly  16  and booster compressor  24  through a second drive shaft  34  that is coupled coaxially with first drive shaft  32 . After driving low-pressure turbine  14 , the combustion products leave turbine engine assembly  10  through an exhaust nozzle  36  to provide propulsive jet thrust.  
         [0015]     Counter-rotating fan assembly  16  includes a forward fan assembly  50  and an aft fan assembly  52  disposed about longitudinal centerline axis  11 . The terms “forward fan” and “aft fan” are used herein to indicate that fan assembly  50  is coupled axially upstream from fan assembly  52 . In the exemplary embodiment, fan assemblies  50  and  52  are positioned at a forward end of core gas turbine engine  12  as illustrated. In an alternative embodiment, fan assemblies  50  and  52  are each positioned at an aft end of core gas turbine engine  12 . Fan assemblies  50  and  52  each include at least one row of rotor blades  60  and  62 , respectively, and are each positioned within a nacelle  64 . Blades  60  and  62  are coupled to respective rotor disks  66  and  68 .  
         [0016]     In the exemplary embodiment, booster compressor  24  includes a plurality of rows of rotor blades  70  that are coupled to a respective rotor disk  72 . In the exemplary embodiment, booster compressor  24  is positioned aft of an inlet guide vane assembly  74  and is coupled to aft fan assembly  52  such that booster compressor  24  rotates at a rotational speed that is substantially equal to a rotational speed of aft fan assembly  52 . Although booster compressor  24  is shown as having only three rows of rotor blades  70 , it should be realized that booster compressor  24  may have a single row of rotor blades  70 , or a plurality of rows of rotor blades  70  that are interdigitated with a plurality of rows of guide vanes  76 . In one embodiment, inlet guide vanes  76  are fixedly coupled to a booster case  78 . In another embodiment, rotor blades  70  are rotatably coupled to rotor disk  72  such that inlet guide vanes  76  are movable during engine operation to facilitate varying a quantity of air channeled through booster compressor  24 . In an alternative embodiment, turbine engine assembly  10  does not include booster compressor  24 .  
         [0017]     In the exemplary embodiment, low-pressure turbine  14  is coupled to forward fan assembly  50  through shaft  34  such that low-pressure turbine  14  and forward fan assembly  50  rotate in a first rotational direction  80 , and aft fan assembly  52  is coupled to low-pressure turbine  14  such that aft fan assembly  52  rotates in an opposite second direction  82 .  
         [0018]      FIG. 2  is a schematic diagram of a portion of counter-rotating fan assembly  16  shown in  FIG. 1 .  FIG. 3  is a schematic diagram of a portion of the counter-rotating fan assembly  16  shown in  FIG. 2  including an exemplary mechanical fuse  200 . In the exemplary embodiment, counter-rotating fan assembly  16  also includes a gearbox  100  that is coupled between aft fan assembly  52  and second drive shaft  34  to facilitate rotating aft fan assembly  52  in a second opposite direction  82  than forward fan assembly  50 .  
         [0019]     In one embodiment, gearbox assembly  100  has a gear ratio of approximately 2 to 1 such that forward fan assembly  50  rotates at a rotational speed that is approximately twice the rotational speed of aft fan assembly  52 . In another embodiment, forward fan assembly  50  rotates with a rotational speed that is between approximately 0.9 and 2.1 times faster than the rotational speed of aft fan assembly  52 . In another embodiment, forward fan assembly  50  rotates at a rotational speed that is approximately 1.5 times faster than the rotational speed of aft fan assembly  52 . In a further embodiment, forward fan assembly  50  rotates at a rotational speed that is approximately 0.67 times the rotational speed of aft fan assembly  52 . Accordingly, in the exemplary embodiment, forward fan assembly  50  rotates at a rotational speed that is faster than the rotational speed of aft fan assembly  52 . In an alternative embodiment, forward fan assembly  50  rotates at a rotational speed that is slower than the rotational speed of aft fan assembly  52 . In the exemplary embodiment, gearbox  100  is a planetary gearbox that substantially radially circumscribes shaft  34  and includes a support structure  102 , at least one gear  103  coupled within support structure  102 , an input  104 , and an output  106 .  
         [0020]     In the exemplary embodiment, turbine engine assembly  10  also includes a first fan bearing assembly  110 , a second fan bearing assembly  120 , a third fan bearing assembly  130 , and a fourth fan bearing assembly  140 . First fan bearing assembly  110  includes a bearing race  112  and a rolling element  114  coupled within bearing race  112 . Second fan bearing assembly  120  includes a bearing race  122  and a rolling element  124  coupled within bearing race  122 . In the exemplary embodiment, fan bearing assemblies  110  and  120  are each thrust bearings that facilitate maintaining forward fan assembly  50  and aft fan assembly  52 , respectively, in a relatively fixed axial position. Third fan bearing assembly  130  includes a bearing race  132  and a rolling element  134  that is coupled within bearing race  132 . Fourth fan bearing assembly  140  includes a bearing race  142  and a rolling element  144  that is coupled within bearing race  142 . In the exemplary embodiment, fan bearing assemblies  130  and  140  are each roller bearings that facilitate providing rotational support to aft fan assembly  52  such that aft fan assembly  52  can rotate freely with respect to forward fan assembly  50 . Accordingly, fan bearing assemblies  130  and  140  facilitate maintaining aft fan assembly  52  in a relatively fixed radial position within counter-rotating fan assembly  16 .  
         [0021]     In the exemplary embodiment, gearbox support structure  102  is coupled to a stationary component. More specifically, and in the exemplary embodiment, fan bearing assembly  120  includes a rotating inner race  126  and a stationary outer race  128  such that rolling element  124  is coupled between races  126  and  128 , respectively. More specifically, in the exemplary embodiment, gearbox input  104  is rotatably coupled to second drive shaft  34  via a drive shaft extension  136  that is splined to drive shaft  34 , and a gearbox output  106  is rotatably coupled to aft fan assembly  52  via an output structure  138 . More specifically, a first end of output structure  138  is splined to gearbox output  106  and a second end of output structure  138  is coupled to drive shaft  168  to facilitate driving aft fan assembly  52 . Outer race  128  facilitates maintaining assembly gearbox  100  in a substantially fixed position within turbine engine assembly  10 .  
         [0022]     Gas turbine engine assembly  12  also includes at least one mechanical fuse  200  that is coupled between drive shaft  34  and gearbox input  104 . More specifically, and in the exemplary embodiment, drive shaft extension  136  includes a first portion  210  and a second portion  212 . First portion  210  is coupled to drive shaft  34  utilizing a plurality of splines  214 , for example, second portion  212  is coupled to gearbox input  104  utilizing a plurality of splines  216 , for example, and first portion  210  is coupled to second portion  212  utilizing a plurality of splines  218 , for example. Accordingly, mechanical fuse  200  is coupled between first and second portions  210  and  212 , respectively, such that drive shaft  34  is coupled to gearbox input  104 .  
         [0023]     In the exemplary embodiment, fuse  200  is approximately disk shaped and includes a radially inner portion  230  that is coupled to input  104  via splines  216  and a radially outer portion  232  that is coupled to first portion  210  via splines  218 . Moreover, fuse  200  has a first thickness  240  proximate radially inner portion  230  and a second thickness  242 , proximate radially outer portion  232 , that is less than first thickness  240 . More specifically, and in the exemplary embodiment, a thickness of disk or fuse  200  gradually decreases from radially inner portion  230  to radially outer portion  232 . In the exemplary embodiment, second thickness  242  is selected such that first portion  230  will separate from second portion  232 , i.e. fuse  200  will break, when fuse  200  is subjected to a load and/or torque between approximately 45% and approximately 55% of the total torque load on the low-pressure turbine drive shaft.  
         [0024]      FIG. 4  is a schematic diagram of a portion of the counter-rotating fan assembly  16  shown in  FIG. 2  including an exemplary mechanical fuse  300 . Gas turbine engine assembly  12  also includes at least one mechanical fuse  300  that is coupled between drive shaft  34  and gearbox input  104 . More specifically, and in the exemplary embodiment, drive shaft extension  136  includes a first portion  210  and a second portion  212 . First portion  210  is coupled to drive shaft  34  utilizing a plurality of splines  214 , for example, second portion  212  is coupled to gearbox input  104  utilizing a plurality of splines  216 , for example, and first portion  210  is coupled to second portion  212  utilizing at least one mechanical fuse  300 . Accordingly, mechanical fuse  300  is utilized to coupled first and second portions  210  and  212  together, such that drive shaft  34  is coupled to gearbox input  104 . In the exemplary embodiment, a plurality of fuses are utilized to couple first and second portions  210  and  212  together.  
         [0025]     During operation, as second drive shaft  34  rotates, second drive shaft  34  causes gearbox input  104  to rotate, which subsequently rotates gearbox output  106 . Because bearing outer race  128  is coupled to aft fan assembly  52 , second drive shaft  34  causes aft fan assembly  52  to rotate via gearbox  100  in an opposite second direction  82  than forward fan assembly  50 . In the exemplary embodiment, gearbox  100  is located within a sump  160  defined between aft fan drive shaft  68  and a structural support member  162  configured to support aft fan assembly  52 . During operation, gearbox  100  is at least partially submerged within lubrication fluid contained in sump  160 . As such, gearbox  100  is facilitated to be continuously lubricated during engine operation.  
         [0026]     Moreover, during operation of engine assembly  10 , an imbalance of engine  10  may cause high radial forces to be applied to aft fan assembly  52  (shown in  FIG. 1 ). To compensate for the relatively high radial stresses and to facilitate ensuring continued engine operation, the mechanical fuse  200  and/or  300  may break such that forward fan assembly  50  continues to operate.  
         [0027]     The gas turbine engine assembly described herein includes a counter-rotating (CR) fan assembly having a geared single rotation (SR) low-pressure turbine. The assembly facilitates reducing at least some of the complexities associated with known counter-rotating low-pressure turbines. More specifically, the gas turbine engine assembly described herein includes a front fan that is rotatably coupled to a single rotation low-pressure turbine, and an aft fan assembly and booster assembly that are rotatably coupled together, and driven by, the low-pressure turbine via a gearbox. The aft fan assembly and booster assembly are driven at the same speed, which, in the exemplary embodiment, is approximately one-half the front fan speed. Additionally, the gas turbine engine assembly described herein is configured such that approximately 40% of power generated by the low-pressure turbine is transmitted through the gearbox to the aft fan assembly to facilitate reducing gear losses.  
         [0028]     Moreover, the gas turbine engine assembly described herein includes a mechanical fuse that is formed by a circled spline and arm assembly that is coupled between the aft fan assembly and the low-pressure turbine drive shaft to facilitate protecting the drive shaft against gear lock up. More specifically, the mechanical fuse described herein will break in the unlikely event that full LPT torque is transmitted to the gearbox during gearbox seizure. Since the gearbox drives the aft fan assembly and booster assembly, the over torque condition with fuse activation will not affect the front fan assembly. As a result, the engine is still capable of producing a useful amount of thrust. More specifically, in the event of a gearbox failure, i.e. the aft fan assembly ceases to rotate, the front fan assembly will continue to operate since it is directly driven by the low-pressure turbine.  
         [0029]     As a result, the gas turbine engine assembly described herein facilitates increasing fan efficiency, reducing fan tip speed, and/or reducing noise. Moreover, since the gas turbine engine assembly described herein does not include a counter-rotating low-pressure turbine to drive the counter-rotating fan assemblies, various components such as, but not limited to, an outer rotating spool, a rotating rear frame, a second low-pressure turbine shaft, and a low-pressure turbine outer rotating seal are eliminated, thus reducing the overall weight of the gas turbine engine assembly. Moreover, in some gas turbine engine applications a mid turbine frame may be eliminated utilizing the methods and apparatuses described herein.  
         [0030]     Exemplary embodiments of a gas turbine engine assembly that includes a gearbox coupled to a fan assembly 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 gearbox described herein can also be used in combination with other known gas turbine engines that include a forward and an aft fan assembly.  
         [0031]     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.