Abstract:
A mid-turbine frame located in a gas turbine engine axially aft of a high-pressure turbine and fore of a low-pressure turbine includes an outer frame case, an inner frame case, and at least a first spoke connecting the outer frame case to the inner frame case. The first spoke includes a tie rod having a first threaded surface and a connector having a second threaded surface. The first and second threaded surfaces overlap partially but not completely.

Description:
BACKGROUND 
     The present disclosure relates generally to a gas turbine engine, and in particular to a mid-turbine frame (MTF) included in a gas turbine engine. 
     A mid-turbine frame (MTF) is positioned between a high pressure turbine stage and a low pressure turbine stage of a gas turbine engine. The MTF supports one or more bearings and transfers bearing loads from an inner portion of the gas turbine engine to an outer engine frame. The MTF also serves to route air from the high pressure turbine stage to the low pressure turbine stage. 
     SUMMARY 
     A gas turbine engine includes a combustor, a first turbine section in fluid communication with the combustor, a second turbine section in fluid communication with the first turbine section, and a mid-turbine frame located axially between the first turbine section and the second turbine section. The mid-turbine frame includes an outer frame case, an inner frame case, and at least a first spoke connecting the outer frame case to the inner frame case. The first spoke includes a tie rod having a first threaded surface and a connector having a second threaded surface. The first and second threaded surfaces overlap partially but not completely. 
     Another embodiment is a mid-turbine frame located in a gas turbine engine axially aft of a high-pressure turbine and fore of a low-pressure turbine. The mid-turbine frame includes an outer frame case, an inner frame case, and at least a first spoke connecting the outer frame case to the inner frame case. The first spoke includes a tie rod having a first threaded surface and a connector having a second threaded surface. The first and second threaded surfaces overlap partially but not completely. 
     Another embodiment is a method of assembling a mid-turbine frame for use in a gas turbine engine axially aft of a high-pressure turbine and fore of a low-pressure turbine. The method includes positioning an outer frame case radially outward of an inner frame case, attaching the outer frame case to the inner frame case via a plurality of spokes, and tightening the plurality of spokes to center the inner frame case with the outer frame case. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a gas turbine engine according to an embodiment of the present invention. 
         FIG. 2A  is a schematic perspective view of one embodiment of a mid-turbine frame (MTF) located in the gas turbine engine. 
         FIG. 2B  is a schematic perspective view of an alternative embodiment of the mid-turbine frame (MTF) of  FIG. 2A . 
         FIG. 3A  is a cross-sectional view of one embodiment of the mid turbine frame (MTF) taken along line  3 - 3  of  FIG. 2A . 
         FIG. 3B  is a cross-sectional view of another embodiment of the mid turbine frame (MTF) taken along line  3 - 3  of  FIG. 2A . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates an example gas turbine engine  20  that includes fan section  22 , compressor section  24 , combustor section  26  and turbine section  28 . Alternative engines might include an augmenter section (not shown) among other systems or features. Fan section  22  drives air along bypass flow path B while compressor section  24  draws air in along core flow path C where air is compressed and communicated to combustor section  26 . In combustor section  26 , air is mixed with fuel and ignited to generate a high pressure exhaust gas stream that expands through turbine section  28  where energy is extracted and utilized to drive fan section  22  and compressor section  24 . 
     Although the disclosed non-limiting embodiment depicts a turbofan gas turbine engine, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines; for example a turbine engine including a three-spool architecture in which three spools concentrically rotate about a common axis and where a low spool enables a low pressure turbine to drive a fan via a gearbox, an intermediate spool that enables an intermediate pressure turbine to drive a first compressor of the compressor section, and a high spool that enables a high pressure turbine to drive a high pressure compressor of the compressor section. 
     The example engine  20  generally includes low speed spool  30  and high speed spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that various bearing systems  38  at various locations may alternatively or additionally be provided. 
     Low speed spool  30  generally includes inner shaft  40  that connects fan  42  and low pressure (or first) compressor section  44  to low pressure (or first) turbine section  46 . Inner shaft  40  drives fan  42  through a speed change device, such as geared architecture  48 , to drive fan  42  at a lower speed than low speed spool  30 . High-speed spool  32  includes outer shaft  50  that interconnects high pressure (or second) compressor section  52  and high pressure (or second) turbine section  54 . Inner shaft  40  and outer shaft  50  are concentric and rotate via bearing systems  38  about engine central longitudinal axis A. 
     Combustor  56  is arranged between high pressure compressor  52  and high pressure turbine  54 . In one example, high pressure turbine  54  includes at least two stages to provide a double stage high pressure turbine  54 . In another example, high pressure turbine  54  includes only a single stage. As used herein, a “high pressure” compressor or turbine experiences a higher pressure than a corresponding “low pressure” compressor or turbine. 
     The example low pressure turbine  46  has a pressure ratio that is greater than about 5. The pressure ratio of the example low pressure turbine  46  is measured prior to an inlet of low pressure turbine  46  as related to the pressure measured at the outlet of low pressure turbine  46  prior to an exhaust nozzle. 
     Mid-turbine frame  57  of engine static structure  36  is arranged generally between high pressure turbine  54  and low pressure turbine  46 . Mid-turbine frame  57  further supports bearing systems  38  in turbine section  28  as well as setting airflow entering low pressure turbine  46 . 
     The core airflow C is compressed by low pressure compressor  44  then by high pressure compressor  52  mixed with fuel and ignited in combustor  56  to produce high speed exhaust gases that are then expanded through high pressure turbine  54  and low pressure turbine  46 . Mid-turbine frame  57  includes airfoils/vanes  59 , which are in the core airflow path and function as an inlet guide vane for low pressure turbine  46 . Utilizing vanes  59  of mid-turbine frame  57  as inlet guide vanes for low pressure turbine  46  decreases the length of low pressure turbine  46  without increasing the axial length of mid-turbine frame  57 . Reducing or eliminating the number of vanes in low pressure turbine  46  shortens the axial length of turbine section  28 . Thus, the compactness of gas turbine engine  20  is increased and a higher power density may be achieved. 
     The disclosed gas turbine engine  20  in one example is a high-bypass geared aircraft engine. In a further example, gas turbine engine  20  includes a bypass ratio greater than about six (6), with an example embodiment being greater than about ten (10). The example geared architecture  48  is an epicyclical gear train, such as a planetary gear system, star gear system or other known gear system, with a gear reduction ratio of greater than about 2.3. 
     In one disclosed embodiment, gas turbine engine  20  includes a bypass ratio greater than about ten (10:1) and the fan diameter is significantly larger than an outer diameter of low pressure compressor  44 . It should be understood, however, that the above parameters are only exemplary of one embodiment of a gas turbine engine including a geared architecture and that the present disclosure is applicable to other gas turbine engines. 
     A significant amount of thrust is provided by bypass flow B due to the high bypass ratio. Fan section  22  of engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft., with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (“TSFC”)”—is the industry standard parameter of pound-mass (lbm) of fuel per hour being burned divided by pound-force (lbf) of thrust the engine produces at that minimum point. 
     “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.50. In another non-limiting embodiment the low fan pressure ratio is less than about 1.45. 
     “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/518.7) 0.5 ]. The “Low corrected fan tip speed”, as disclosed herein according to one non-limiting embodiment, is less than about 1150 ft/second. 
     The example gas turbine engine includes fan  42  that comprises in one non-limiting embodiment less than about 26 fan blades. In another non-limiting embodiment, fan section  22  includes less than about 20 fan blades. Moreover, in one disclosed embodiment low pressure turbine  46  includes no more than about 6 turbine rotors schematically indicated at  34 . In another non-limiting example embodiment low pressure turbine  46  includes about 3 turbine rotors. A ratio between number of fan blades  42  and the number of low pressure turbine rotors is between about 3.3 and about 8.6. The example low pressure turbine  46  provides the driving power to rotate fan section  22  and therefore the relationship between the number of turbine rotors  34  in low pressure turbine  46  and number of blades  42  in fan section  22  disclose an example gas turbine engine  20  with increased power transfer efficiency. 
       FIG. 2A  is a schematic perspective view of one embodiment of mid turbine frame (MTF)  57 . The schematic view shown in  FIG. 2A  is high level conceptual view and is intended to illustrate relative positioning of various components, but not actual shape of various components. MTF  57  includes outer frame case  62 , inner frame case  64 , and a plurality of hollow spokes  65 . Outer frame case  62  includes outer diameter surface  66 . Inner frame case  64  includes outer diameter surface  70  and inner diameter surface  72 . In the embodiment shown in  FIG. 2A , six hollow spokes  65  are distributed evenly around the circumference of radial inner case  64  to provide structural support between inner frame case  64  and outer frame case  62 . In the illustrated embodiment, each of the hollow spokes  65  is directly opposite (i.e. 180 degrees from) another of the hollow spokes  65 . In alternative embodiments, MTF  57  can have an even number of hollow spokes greater than or less than six. 
     Inner frame case  64  supports the rotor assembly via bearing assemblies  38  (shown in  FIG. 1 ), and distributes the force from inner frame case  64  to outer frame case  62  via the plurality of hollow spokes  65 . Attachment of hollow spokes  65  to outer frame case  62  is provided at a plurality of bosses  75  located circumferentially around outer diameter surface  66  of outer frame case  62 . In one embodiment, attachment of hollow spokes  65  at bosses  75  may be secured by a retaining nut (shown in  FIGS. 3A and 3B ) that allows hollow spokes  65  to be tensioned. Hollow spokes  65  can be tensioned via a threaded connection so as to remain in tension during substantially all operating conditions of gas turbine engine  20 , as further discussed below. Apertures  76  formed in each of the plurality of bosses  75  allow cooling air to be distributed into a hollow portion of each of the hollow spokes  65 . In this way, cooling airflow is directed from the outer diameter through the hollow portions of cooled spokes  65  towards inner frame case  64 . This cooling air can function to cool hollow spokes  65  and also to cool components radially inward of inner frame case  64 , such as bearings  38 . 
       FIG. 2B  is a schematic perspective view of another embodiment of mid turbine frame (MTF)  57 . The schematic view shown in  FIG. 2B  is high level conceptual view and is intended to illustrate relative positioning of various components, but not actual shape of various components. In the embodiment shown in  FIG. 2B , seven hollow spokes  65  are distributed evenly around the circumference of radial inner case  64  to provide structural support between inner frame case  64  and outer frame case  62 . In the illustrated embodiment, no two hollow spokes  65  are directly opposite one-another. In alternative embodiments, MTF  57  can have an odd number of hollow spokes greater than or less than seven. 
       FIG. 3A  is a cross-sectional view of an embodiment of MTF  57  taken along line  3 - 3  of  FIG. 2A . Hollow spoke  65 A is one embodiment of one of hollow spokes  65  shown in  FIGS. 2A and 2B . Hollow spoke  65 A extends from outer frame case  62  through airfoil  59  to inner frame case  64 . Airfoil  59  extends from outer platform  78  to inner platform  80 . In the illustrated embodiment, airfoil  59 , outer platform  78 , and inner platform  80  are integrally formed, and are all positioned radially inward of outer frame case  62  and radially outward of inner frame case  64 . Airfoil  59 , outer platform  78 , and inner platform  80  define a portion of core flowpath C at MTF  57 . Airfoil  59  extends axially from leading edge  82  to trailing edge  84 . Airfoil  59  is oblong so as to be longer in the axial direction than in the circumferential direction. Airfoil  59  has a hollow interior  86 , which is also relatively narrow in a circumferential direction. 
     In the illustrated embodiment, hollow spoke  65 A includes tie rod  90 A and retaining nut  92 . Tie rod  90 A is an elongated hollow tube that includes threaded surface  94  at a radially outer end and flange  96  at a radially inner end. Threaded surface  94  is on outer surface  98  of tie rod  90 A. Inner passage surface  100  of tie rod  90 A defines a hollow passage through tie rod  90 A. Tie rod  90 A tapers along its length from flange  96  at its radially inner end to threaded surface  94  at its radially outer end. 
     Retaining nut  92  includes threaded surface  102  at a radially inner end of retaining nut  92  and flange  104  at a radially outer end of retaining nut  92 . Threaded surface  102  is on inner surface  106  of retaining nut  92 . Flange  104  extends outward from outer surface  108  of retaining nut  92 . 
     In the illustrated embodiment, flange  96  of tie rod  90 A abuts against inner frame case  64  so that inner passage surface  100  aligns with hole  110 A of inner frame case  64 . Flange  96  is attached to inner frame case  64  via bolts  112 . Retaining nut  92  extends through hole  114  in outer frame case  62  such that flange  104  abuts against outer diameter surface  66  of outer frame case  62 . Flange  104  is attached to outer frame case  62  via bolt  116 . Bolt  116  extends through flange  104  into outer frame case  62 . Tie rod  90 A is threaded into retaining nut  92  so as to attach tie rod  90 A to retaining nut  92 . In the illustrated embodiment, a portion but not all of threaded surface  94  overlaps with a portion but not all of threaded surface  102 . 
     During assembly, tie rod  90 A is inserted through hollow interior  86  of airfoil  59  in a direction from radially inward to radially outward. Inner frame case  64  is then positioned radially inward of tie rod  90 A and attached to tie rod  90 A by bolts  112 . Retaining nut  92  is then inserted through hole  114  and threadedly engaged with tie rod  90 A. Retaining nut  92  can be tightened, as desired, in a manner described below. Once retaining nut  92  is suitably tightened on tie rod  90 A, bolt  116  is inserted to fix retaining nut  92  to outer frame case  62  so as to prevent retaining nut  92  from rotating and loosening. 
     Because threaded surface  94  overlaps with threaded surface  102  only partially, the threaded connection between retaining nut  92  and tie rod  90 A is variable. Retaining nut  92  does not bottom out at any particular point when threaded on tie rod  90 A. This allows retaining nut  92  to be threaded on tie rod  90 A to an extent determined during assembly, not predetermined prior to assembly. This allows hollow spoke  65 A, and MTF  57  in general, to be relatively insensitive to manufacturing tolerances. 
       FIG. 3B  is a cross-sectional view of another embodiment of MTF  57  taken along line  3 - 3  of  FIG. 2A . In the embodiment illustrated in  FIG. 3B , hollow spoke  65 B, tie rod  90 B, and hole  110 B replace hollow spoke  65 A, tie rod  90 A, and hole  110 A (shown in  FIG. 3A ), respectively. Tie rod  90 B differs from tie rod  90 A in that tie rod  90 B does not substantially taper along its length from threaded surface  120  at its radially inner end to threaded surface  94  at its radially outer end. Tie rod  90 B is an elongated hollow and slender tube. Tie rod  90 B extends through hole  110 B of inner frame case  64 , with abutment surface  122  of tie rod  90 B abutting abutment surface  124  of inner frame case  64 . Retaining nut  126  has threaded surface  128  engaged with threaded surface  120  of tie rod  90 B. Retaining nut  126  and threaded surface  120  are positioned radially inward of inner frame case  64 . Threaded surface  94  is positioned radially between inner frame case  64  and outer frame case  62 . Thus, hollow spoke  65 B extends through hole  114  in outer frame case  62  and through hole  110 B in inner frame case  64 . 
     During assembly, airfoil  90  is positioned between outer frame case  62  and inner frame case  64  such that hole  114 , hollow interior  86 , and hole  110 B are substantially aligned. Then, tie rod  90 B is inserted through hole  114 , hollow interior  86 , and hole  110 B in a direction from radially outward to radially inward. Tie rod  90 B is slid into hole  110 B until abutment surface  122  bottoms out against abutment surface  124 . Retaining nut  126  is then threaded on tie rod  90 B until retaining nut  126  bottoms out against inner frame case  64 . Retaining nut  126  is tightened to fixedly attach tie rod  90 B to inner frame case  64 . Retaining nut  92  is then inserted through hole  114  and threadedly engaged with tie rod  90 B. Retaining nut  92  can be tightened, as desired, in a manner described below. Once retaining nut  92  is suitably tightened on tie rod  90 B, bolt  116  is inserted to fix retaining nut  92  to outer frame case  62  so as to prevent retaining nut  92  from rotating and loosening. 
     Because threaded surface  94  overlaps with threaded surface  102  only partially, the threaded connection between retaining nut  92  and tie rod  90 B is variable. Retaining nut  92  does not bottom out at any particular point when threaded on tie rod  90 B. This allows retaining nut  92  to be threaded on tie rod  90 B to an extent determined during assembly, not predetermined prior to assembly. This allows hollow spoke  65 B, and MTF  57  in general, to be relatively insensitive to manufacturing tolerances. 
     Though  FIGS. 3A and 3B  each show a single hollow spoke  65  (hollow spokes  65 A and  65 B), they are one of several hollow spokes  65  spaced circumferentially around MTF  57  (shown in  FIGS. 2A and 2B ). As each of hollow spokes  65  are tightened, inner frame case  64  can move slightly with respect to outer frame case  62 . Hollow spokes  65  can be tightened in a manner that they collectively center inner frame case  64  with respect to outer frame case  62  relatively precisely. 
     For example, in one embodiment, outer frame case  62  and inner frame case  64  can be designed to be centered and coaxial within 5 thou (5 thousandths of an inch or 0.005 inch or 5 mils or 0.127 mm, millimeter). If each of outer frame case  62 , inner frame case  64 , tie rod  90 , and retaining nut  92  are also manufactured to a tolerance of +/−5 thou (0.127 mm), they could combine to have a variance of +/−20 thou (0.508 mm). However, because the threaded connection between retaining nut  92  and tie rod  90  is variable, retaining nut  92  can be tightened or loosened as necessary to account for the combined variance due to tolerances of individual parts. This allows tie rod  90 , retaining nut  92 , and other components of MTF  57  to be manufactured with relatively loose individual tolerances, yet still combine to form a structure with relatively tight tolerances. This can reduce the overall cost of manufacture of MTF  57 . In other embodiments, each of outer frame case  62 , inner frame case  64 , tie rod  90 , and retaining nut  92  can be manufactured to a tolerance of +/−5 thou (0.127 mm) or greater, while outer frame case  62  can be centered with inner frame case  64  with a tolerance of +/−5 thou (0.127 mm) or less. In still other embodiments, various components can have tolerances not specifically listed above. 
     Retaining nuts  92  can be rotated and tightened on tie rods  90 A and  90 B such that hollow spokes  65  have a sufficient initial tension so that hollow spokes  65  remain in tension during all operating conditions. For example, MTF  57  can experience a wide variation in temperatures ranging between relatively cool temperatures when gas turbine engine  20  is not operating and relatively hot temperatures when gas turbine engine  20  is operating at a high speed. Because MTF  57  includes components having different shapes, made of different materials, and exposed to different temperatures, such components can expand and contract at different rates when heated and cooled. For example, if an operating condition causes inner frame case  64  to expand more than outer frame case  62 , then the tension on hollow spoke  65  can be reduced. If tension on hollow spoke  65  were reduced to zero (reaching or passing a “null line”), MTF  57  could become imbalanced and cause substantial damage to bearings  38  (shown in  FIG. 1 ) and/or other components on gas turbine engine  20 . By tightening hollow spokes  65  to a tension that allows them to remain in tension during substantially all operating conditions, gas turbine engine  20  can avoid damage caused by a loss of tension in hollow spokes  65 . Moreover, because hollow spokes  65  remain in tension, they are not subjected to damage caused by mechanical fatigue from cycling between tension and compression. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. For example, retaining nut  92  and tie rods  65 A and  65 B can be sized and shaped as appropriate for a particular application. Additionally, MTF  57  can be used in a gas turbine engine different than gas turbine engine  20 , and can be modified as appropriate for that engine.