Abstract:
A method for servicing a gas turbine engine includes providing access from a forward section of the gas turbine engine to a gearbox contained within a bearing compartment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present disclosure is a continuation-in-part application of U.S. patent application Ser. No. 12/622,535, filed Nov. 20, 2009. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to a gas turbine engine, and in particular, to a case structure therefor. 
         [0003]    Epicyclic gearboxes with planetary or star gear trains may be used in gas turbine engines for their compact designs and efficient high gear reduction capabilities. Planetary and star gear trains generally include three gear train elements: a central sun gear, an outer ring gear with internal gear teeth, and a plurality of planet gears supported by a planet carrier between and in meshed engagement with both the sun gear and the ring gear. The gear train elements share a common longitudinal central axis, about which at least two rotate. 
         [0004]    During flight, light weight structural cases deflect with aero and maneuver loads which may cause significant deflection commonly known as backbone bending of the engine. This deflection may result in some misalignment of the gear train elements which may lead to efficiency losses and potential reduced life. Management of the deflections of the static and rotating components as well as minimization of heat loads facilitate successful engine architectures. 
       SUMMARY 
       [0005]    A gas turbine engine according to an exemplary aspect of the present disclosure includes a low pressure compressor along an axis, a first bearing system which at least partially supports an inner shaft along the axis, a second bearing system which at least partially supports an outer shaft along the axis; and a low pressure compressor hub mounted to the inner shaft, the low pressure compressor hub extends to the low pressure compressor between the first bearing system and the second bearing support. 
         [0006]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the low pressure compressor hub may include a frustro-conical web which extends between the first bearing system and the second bearing support. 
         [0007]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the frustro-conical web may extend at least partially around the first bearing support. 
         [0008]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the low pressure compressor may be radially outboard of the first bearing support. Additionally or alternatively, the low pressure compressor hub may be angled toward the low pressure compressor. 
         [0009]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the low pressure compressor hub may be mounted to a second stage disk of the low pressure compressor. Additionally or alternatively, the low pressure compressor may include three stages. 
         [0010]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the inner shaft may drive a fan through a geared architecture. 
         [0011]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the first bearing system may be mounted to a front center body case structure, the front center body case structure may define a core flow path for a core airflow. 
         [0012]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the second bearing system may be mounted to an intermediate case structure, the intermediate case structure may be mounted to the front center body case structure to continue the core flow path. 
         [0013]    A gas turbine engine according to another exemplary aspect of the present disclosure includes a front center body case structure, a geared architecture at least partially supported by the front center body case structure, a first bearing system mounted to the front center body case structure to rotationally support an inner shaft, a coupling shaft mounted to the inner shaft and the geared architecture, the coupling shaft at least partially supported by the first bearing support. 
         [0014]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the inner shaft may drive a fan through the geared architecture. 
         [0015]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, may further comprise an outer shaft which may at least partially surround the inner shaft, the outer shaft drives a high pressure compressor. 
         [0016]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, a second bearing system may at least partially support an outer shaft. 
         [0017]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, a low pressure compressor hub may be mounted to the inner shaft, the low pressure compressor hub may extend to a low pressure compressor between the first bearing system and the second bearing support. 
         [0018]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the low pressure compressor may include three stages, the low pressure compressor hub may be mounted to a second stage disk of the low pressure compressor. 
         [0019]    A gas turbine engine according to another exemplary aspect of the present disclosure includes a front center body case structure along an engine axis, the front center body case structure defines a core flow path, a low pressure compressor along the core flow path, an intermediate case structure mounted aft of the front center body case structure along the engine axis, a first bearing system mounted to the front center body case structure to at least partially support an inner shaft for rotation about the engine axis, a second bearing system mounted to the intermediate case structure to at least partially support an outer shaft for rotation about the engine axis, a low pressure compressor hub mounted to the inner shaft, the low pressure compressor hub extends to the low pressure compressor between the first bearing system and the second bearing support. 
         [0020]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the front center body case structure may be downstream of a fan. 
         [0021]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the inner shaft may drive the fan through a geared architecture. 
         [0022]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the geared architecture maybe at least partially supported by the front center body case structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
           [0024]      FIG. 1  is a schematic cross-section of a forward portion of a gas turbine engine; 
           [0025]      FIG. 2  is a perspective cross-sectional view of a bearing compartment including a first tapered roller bearing, a second tapered roller bearing and a bellows spring; 
           [0026]      FIG. 3  is an enlarged cross section of the bellows spring of  FIG. 2 ; 
           [0027]      FIG. 4  is a schematic cross-section of a gas turbine engine; 
           [0028]      FIG. 5  is an enlarged schematic cross-section of a sectional of the gas turbine engine which illustrates a front center body case structure; 
           [0029]      FIG. 6  is a schematic block diagram of a gas turbine engine with the inventive architecture; 
           [0030]      FIG. 7  is a schematic block diagram of a RELATED ART gas turbine engine with the inventive architecture; and 
           [0031]      FIG. 8  is an enlarged schematic cross-section of the  FIG. 5  sectional of the gas turbine engine which illustrates a load path within the front center body case structure. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  shows a forward section of gas turbine engine  10  above engine centerline C L  of gas turbine engine  10 . Gas turbine engine  10  includes bearing compartment  12 , first and second tapered roller bearings  14 A and  14 B, fan shaft  16 , bearing system  18 , bellows spring  20 , fan hub  22 , nut  23 , fan blades  24 , fan nose  26 , engine shaft  28 , fan drive gear system  30 , compressor section  32 , guide vanes  34 , and engine case  36 . 
         [0033]    Bearing compartment  12  is disposed adjacent fan shaft  16  and contains first and second tapered roller bearings  14 A and  14 B therein. Fan shaft  16  rotates about an axis that aligns with engine centerline axis C L  and is supported on tapered roller bearings  14 A and  14 B. Bearing compartment  12  is bounded by fan shaft  16  and bearing system  18  which connects to the tapered roller bearings  14 A and  14 B. Bearing system  18  extends to connect to a non-rotational frame such as an engine case of gas turbine engine  10 . Bellows spring  20  is disposed in bearing compartment  12  adjacent first tapered roller bearing  14 A and second tapered roller bearing  14 B. Bellows spring  20  applies a preload to both first tapered roller bearing  14 A and second tapered roller bearing  14 B. Nut  23  is positioned adjacent the fan hub  22  and applies a clamping force to the radially inner race portion of first tapered roller bearing  14 A and the inner race portion of second tapered roller bearing  14 B. 
         [0034]    Fan shaft  16  connects to and turns fan blades  24  through fan hub  22 . Fan hub  22  also connects to fan nose  26 . Fan shaft  16  connects to engine shaft  28  via fan drive gear system  30 . Compressor section  32  is disposed radially outward of engine centerline C L  and is connected to engine shaft  28 . Guide vanes  34  are disposed radially outward of compressor section  32  and are rotatable relative to engine case  36 . 
         [0035]    The operational principles of gas turbine engine  10  are well known in the art, and therefore, will not be discussed in great detail. As illustrated in  FIG. 1 , gas turbine engine  10  comprises a high bypass ratio geared turbofan engine. In other embodiments, gas turbine engine  10  can comprise another type of gas turbine engine used for aircraft propulsion or power generation. Similarly, bearing compartment  12  can comprise any bearing compartment in engine  10 . 
         [0036]    Fan shaft  16  and compressor section  32  are connected to a turbine section (not shown) through engine shaft  28 . Inlet air A enters engine  10  whereby it is divided into streams of a primary air A P  and a secondary air A S  after passing through the fan blades  18 . The fan blades  24  are rotated by turbine section (not shown) of engine  10  through engine shaft  28  to accelerate the secondary air A S  (also known as bypass air) through exit guide vanes  34 , thereby producing a significant portion of the thrust output of engine  10 . The primary air A P  (also known as gas path air) is directed into compressor section  32 . Compressor section  32  works together to incrementally increase the pressure and temperature of primary air A P . 
         [0037]      FIG. 2  shows a perspective cross-sectional view of bearing compartment  12  including first tapered roller bearing  14 A, second tapered roller bearing  14 B, and bellows spring  20 . Additionally, bearing compartment  12  includes seal plate  38 , bearing spacer  40 , gear  42 , secondary sleeve  44 , and squeeze film damper system  46 . First and second tapered roller bearings  14 A and  14 B include inner races  48 A and  48 B, roller elements  50 A and  50 B, and outer races  52 A and  52 B, respectively. Also shown are shoulder  54  of bearing system  18  and shim  56 . 
         [0038]    Within bearing compartment  12 , seal plate  38  abuts a forward portion of (as defined by the direction of primary air Ap flow within the gas turbine engine  10 ) first tapered roller bearing  14 A. Seal plate  38  comprises a portion of the carbon sealing system and is disposed adjacent inner race  48 A. Bearing spacer  40  abuts both inner races  48 A and  48 B to provide necessary spacing between first and second tapered roller bearings  14 A and  14 B. Gear  42  is contacted by inner race  48 B of second tapered roller bearing  14 B and connects to a shoulder portion of fan shaft  16 . In the embodiment shown in  FIG. 2 , secondary sleeve  44  is disposed between outer race  52 A of first tapered roller bearing  14 A and bearing system  18 . Tapered roller bearings  14 A and  14 B can also be supported by squeeze film damper system  46  (of which only seals are shown) disposed between one or more of the tapered roller bearings  14 A and  14 B and bearing system  18 . Squeeze film damper systems such as the one disclosed herein are well known in the art and are used to shift critical speeds and/or to increase the dynamic stability of a rotor-bearing system. 
         [0039]    In particular, first and second tapered roller bearings  14 A and  14 B have inner races  48 A and  48 B that are clamped or otherwise affixed to fan shaft  16 . Inner races  48 A and  48 B have radially outward surfaces (raceways) that interface with roller elements  50 A and  50 B, respectively. Outer races  52 A and  52 B interface with roller elements  50 A and  50 B, respectively, and are mounted to bearing system  18 . In the embodiment shown in  FIG. 2 , outer race  52 A of first tapered roller bearing  14 A is constrained radially and tangentially but can move axially relative to secondary sleeve  44 , bearing system  18 , and portions of squeeze film damper system  46 . This allows roller element  50 A to remain in contact with inner raceway of outer race  52 A. Outer race  52 B of second tapered roller bearing  14 B is fastened to bearing system  18 . First and second tapered roller bearings  14 A and  14 B are retained by bearing system  18 , which reacts loads back through to the engine case  36 . 
         [0040]    In one embodiment, a forward end of bellows spring  20  is snapped into an interference fit with outer race  52 A, and an aft end of bellows spring  20  is snapped into an interference fit with shoulder  54  of bearing system  18 . Thus, bellows spring  20  is positioned generally between first tapered roller bearing  14 A and second tapered roller bearing  14 B. At least one shim  56  can be positioned between the aft end of bellows spring  20  and shoulder  54 . Shim  56  allows the spring preload to be accurately set to a desired level without requiring restrictive manufacturing tolerances of bellows spring  20 , bearing system  18 , or other components. 
         [0041]    Nut  23  ( FIG. 1 ) applies a clamping force which reacts through inner race  48 A of first tapered roller bearing  14 A, through bearing spacer  48 , through inner race  48 B of second tapered roller bearing  14 B, and against gear  42  on fan shaft  16 . Bellows spring  20  applies preload to both first tapered roller bearing  14 A and second tapered roller bearing  14 B. In particular, bellows spring  20  applies preload to outer race  52 A and applies preload to bearing system  18  which transfers preload to outer race  52 B of second tapered roller bearing  14 B. 
         [0042]      FIG. 3  shows an enlarged cross section of one embodiment of bellows spring  20 . In the embodiment shown in  FIG. 3 , bellows spring  20  is a resilient member that is shaped as a corrugated single piece annular ring. Bellows spring  20  is comprised of a hardened stainless steel. Bellows spring  20  is lathe turned to produce the corrugated shape shown. As illustrated in  FIG. 3 , bellows spring  20  can have a cross-sectional thickness that is variable as the bellows spring  20  extends axially along an engine centerline C L  ( FIG. 1 ). 
         [0043]    The number of turns (convolutes) of bellows spring  20  should be maximized (as limited by the size of the bearing compartment  12  and manufacturing practicality) to allow the bellows spring  20  to better accommodate different tolerances of components within the bearing compartment  12 . Analytical tools such as commercially available finite element analysis software can be used to simulate stresses on bellows spring  20  in order to optimize its geometry (number of turns, cross-sectional thicknesses, etc.) and performance. In one embodiment, the turns of bellow spring  20  have a modified omega shape, that is each convolute section  58  of bellows spring  20  extends forward and aft of adjacent interconnection sections  60  (i.e., bellows spring  20  has sections  58  which bend forward or aft relative adjacent sections  60 ). Other embodiments can have parallel convolutes to simplify the manufacturing of bellows spring  20 . 
         [0044]    The use of bellows spring  20  to apply preload to first tapered roller bearing  14 A and second tapered roller bearing  14 B allows a single element to be used in the confined space of bearing compartment  12 , thereby saving space and reducing manufacturing costs. Bellows spring  20  is adapted to apply preload to first and second tapered roller bearings  14 A and  14 B in the axial direction along the engine centerline C L  ( FIG. 1 ), and act as a centering spring (i.e., have a radial stiffness with respect to the engine centerline C L ) for the squeeze film damper system  46  ( FIG. 2 ). Bellows spring  20  is accommodating of flexing in the radial direction by first tapered roller bearing  14 A and the second tapered roller bearing  14 B such that bellows spring  20  does not excessively wear on the surfaces of the bearings  14 A and  14 B. 
         [0045]      FIG. 4  schematically illustrates another exemplary gas turbine engine  120 . 
         [0046]    The gas turbine engine  120  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  122 , a compressor section  124 , a combustor section  126  and a turbine section  128 . Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section  122  drives air along a bypass flowpath while the compressor section  124  drives air along a core flowpath for compression and communication into the combustor section  126  then expansion through the turbine section  128 . Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment, 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 such as a three-spool (plus fan) engine wherein an intermediate spool includes an intermediate pressure compressor (IPC) between the LPC and HPC and an intermediate pressure turbine (IPT) between the HPT and LPT. 
         [0047]    The engine  120  generally includes a low spool  130  and a high spool  132  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  136  via several bearing supports  138 . The low spool  130  generally includes an inner shaft  140  that interconnects a fan  142 , a low pressure compressor  144  and a low pressure turbine  146 . The inner shaft  140  drives the fan  142  through a geared architecture  148  to drive the fan  142  at a lower speed than the low spool  130 . An exemplary reduction transmission is an epicyclic transmission, namely a planetary or star gear system. 
         [0048]    The high spool  132  includes an outer shaft  150  that interconnects a high pressure compressor  152  and high pressure turbine  154 . A combustor  156  is arranged between the high pressure compressor  152  and the high pressure turbine  154 . The inner shaft  140  and the outer shaft  150  are concentric and rotate about the engine central longitudinal axis A which is collinear with their longitudinal axes. 
         [0049]    Core airflow is compressed by the low pressure compressor  144  then the high pressure compressor  152 , mixed with the fuel and burned in the combustor  156 , then expanded over the high pressure turbine  154  and low pressure turbine  146 . The turbines  154 ,  146  rotationally drive the respective low spool  130  and high spool  132  in response to the expansion. 
         [0050]    The main engine shafts  140 ,  150  are supported at a plurality of points by bearing supports  138  within the static structure  136 . In one non-limiting embodiment, bearing supports  138  includes a #2 bearing system  138 - 2  located radially inboard of the compressor section  124 . 
         [0051]    With reference to  FIG. 5 , the engine static structure  136  proximate the compressor section  124  generally includes a front center body case structure  160  and an intermediate case structure  162  which mounts aft of the front center body case structure  160 . It should be appreciate that various case structures may alternatively or additionally be provided, yet benefit from the architecture described herein. 
         [0052]    The front center body case structure  160  generally defines an annular core flow path  164 A for the core airflow into the low pressure compressor  144 . The intermediate case structure  162  defines a core flow path  164 B which continues the core flow path  164 A for the core airflow into the high pressure compressor  152  of core flow path  164 C. The core flow path  164 B is generally radially inward of the core flow path  164 A to transition into the radially smaller diameter core flow path  164 C. That is, the core flow path  164 B defines a “wasp waist” gas turbine engine architecture. 
         [0053]    A #2 bearing system  138 - 2  at least partially supports the inner shaft  140  relative to the front center body case structure  160 . A #3 bearing system  138 - 3  generally supports the outer shaft  150  relative the intermediate case structure  162 . That is, the #2 bearing system  138 - 2  at least partially supports the low spool  130  and the #3 bearing system  138 - 3  generally supports the high spool  132 . It should be appreciated that various bearing systems such as thrust bearing structures and mount arrangements will benefit herefrom. 
         [0054]    A flex support  168  provides a flexible attachment of the geared architecture  48  within the front center body case structure  160 . The flex support  168  reacts the torsional loads from the geared architecture  148  and facilitates vibration absorption as well as other support functions. A centering spring  170  which is a generally cylindrical cage-like structural component with a multiple of beams that extend between flange end structures resiliently positions the #2 bearing system  138 - 2  with respect to the low spool  130 . In one embodiment, the beams are double-tapered beams arrayed circumferentially to control a radial spring rate that may be selected based on a plurality of considerations including, but not limited to, bearing loading, bearing life, rotor dynamics, and rotor deflection considerations. 
         [0055]    The gearbox  172  of the geared architecture  148  is driven by the low spool  130  in the disclosed non-limiting embodiment through a coupling shaft  174 . The coupling shaft  174  transfers torque bearing system to the gearbox  172 . The #2 bearing system  138 - 2  s facilitates the segregation of vibrations and other transients from the gearbox  172 . The coupling shaft  174  in the disclosed non-limiting embodiment includes a forward coupling shaft section  176  and an aft coupling shaft section  178 . The forward coupling shaft section  176  includes an interface spline  180  which mates with the gearbox  172 . An interface spline  182  of the aft coupling shaft section  178  connects the coupling shaft  174  to the low spool  130  through, in this non limiting embodiment, a low pressure compressor hub  184  of the low pressure compressor  144 . 
         [0056]    A fan rotor bearing system structure  186  aft of the fan  142  extends radially inward from the front center body case structure  160 . The fan rotor bearing system structure  86  and the front center body case structure  160  define a bearing compartment B. It should be appreciated that various bearing supports  138 - 1  and seals  188  (illustrated schematically and in  FIG. 5 ) may be supported by the fan rotor bearing system structure  186  to contain oil and support rotation of an output shaft  200  which connects with the geared architecture  148  to drive the fan  142 . 
         [0057]    The low pressure compressor hub  184  of the low pressure compressor  144  includes a tubular hub  190  and a frustro-conical web  192 . The tubular hub  190  mounts to the inner shaft  140  through, for example, a splined interface. The tubular hub  190  is adjacent to the #2 bearing system  138 - 2 . The frustro-conical web  192  extends in a forwardly direction from the tubular hub  190  axially between the #2 bearing system  138 - 2  and the #3 bearing system  138 - 3  (also shown in  FIG. 6 ). That is, the frustro-conical web  192  is axially located between the bearing supports  138 - 2 ,  138 - 3 . 
         [0058]    The frustro-conical web  192  mounts to a low pressure compressor rotor  194  of the low pressure compressor  144 . In the disclosed non-limiting embodiment, the frustro-conical web  192  extends between the bearing supports  138 - 2 ,  138 - 3  and mounts to a second stage of a three stage low pressure compressor rotor  194 . It should be appreciated that the frustro-conical web  192  may mount to other stages in other engine architectures and such architectures may include other numbers of stages. 
         [0059]    Locating the low pressure compressor hub  184  between the #2 bearing system  138 - 2  and the #3 bearing system  138 - 3  offers significant advantage to reduce deflection for the geared architecture  48  as compared to a related art architecture such as the example illustrated in  FIG. 7 ; RELATED ART. That is, both end sections of the coupling shaft  174  are tied to the front center body case structure  160  such that relative deflections between the end sections thereof are greatly reduced. This facilitates a more efficient balance of baseline torque, FBO torques, maneuver deflections and the minimization of the overall loads that are translated into the geared architecture  148 . 
         [0060]    Moreover, a relatively less complicated bearing compartment B which facilitates increased manufacturing tolerances is defined to, for example, require fewer seals which minimizes potential oil leak sources and saves weight. 
         [0061]    The architecture further facilitates an efficient load path (L;  FIG. 8 ) for the geared architecture and an overall lower overall heat generation and oil flow. That is, a more compact load path L is defined by the forward center body structure  160  alone. Secondary benefits are reduced oil tank size, reduced cooler sizing and reduce oil quantity in the engine lubrication system. 
         [0062]    It should be understood that like reference numerals identify corresponding or similar elements throughout the several drawings. It should also be understood that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit herefrom. 
         [0063]    Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present invention. 
         [0064]    Although the different examples have specific components shown in the illustrations, embodiments of this invention are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples. 
         [0065]    The foregoing description is exemplary rather than defined by the limitations within. Various non-limiting embodiments are disclosed herein, however, one of ordinary skill in the art would recognize that various modifications and variations in light of the above teachings will fall within the scope of the appended claims. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described. For that reason the appended claims should be studied to determine true scope and content.