# Patent

## Title
Flexible support structure for a geared architecture gas turbine engine

## Abstract

A gas turbine engine includes a flex mount for a Fan Drive Gear System defined by a lateral and transverse stiffness relatonship.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
The present disclosure is a continuation-in-part of U.S. patent application Ser. No. 13/342,508, filed Jan. 3, 2012, which claims priority to U.S. Provisional Patent Application No. 61/494,453, filed Jun. 8, 2011.


BACKGROUND
The present disclosure relates to a gas turbine engine, and more particularly to a flexible support structure for a geared architecture therefor.
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. An advantage of epicyclic gear trains is that a rotary input can be connected to any one of the three elements. One of the other two elements is then held stationary with respect to the other two to permit the third to serve as an output.
In gas turbine engine applications, where a speed reduction transmission is required, the central sun gear generally receives rotary input from the powerplant, the outer ring gear is generally held stationary and the planet gear carrier rotates in the same direction as the sun gear to provide torque output at a reduced rotational speed. In star gear trains, the planet carrier is held stationary and the output shaft is driven by the ring gear in a direction opposite that of the sun gear.
During flight, light weight structural cases deflect with aero and maneuver loads causing significant amounts of transverse deflection commonly known as backbone bending of the engine. This deflection may cause the individual sun or planet gear's axis of rotation to lose parallelism with the central axis. This deflection may result in some misalignment at gear train journal bearings and at the gear teeth mesh, which may lead to efficiency losses from the misalignment and potential reduced life from increases in the concentrated stresses.
SUMMARY
A gas turbine engine according to an exemplary embodiment of the present disclosure includes a fan shaft and a frame which supports the fan shaft. The frame defines a frame lateral stiffness and a frame transverse stiffness. A gear system drives the fan shaft. A flexible support at least partially supports the gear system. The flexible support defines a flexible support lateral stiffness with respect to the frame lateral stiffness and a flexible support transverse stiffness with respect to the frame transverse stiffness. An input coupling to the gear system defines an input coupling lateral stiffness with respect to the frame lateral stiffness and an input coupling transverse stiffness with respect to the frame transverse stiffness.
In a further non-limiting embodiment, the flexible support lateral stiffness is less than the frame lateral stiffness.
In a further non-limiting embodiment of any of the foregoing examples, the flexible support transverse stiffness is less than the frame transverse stiffness.
In a further non-limiting embodiment of any of the foregoing examples, the flexible support lateral stiffness is less than the frame lateral stiffness, and the flexible support transverse stiffness is less than the frame transverse stiffness.
In a further non-limiting embodiment of any of the foregoing examples, the gear system includes a gear mesh that defines a gear mesh lateral stiffness and a gear mesh transverse stiffness.
A gas turbine engine according to an exemplary embodiment of the present disclosure includes a fan shaft and a frame which supports the fan shaft. A gear system drives the fan shaft. The gear system includes a gear mesh that defines a gear mesh lateral stiffness and a gear mesh transverse stiffness. A flexible support at least partially supports the gear system. The flexible support defines a flexible support lateral stiffness with respect to the gear mesh lateral stiffness and a flexible support transverse stiffness with respect to the gear mesh transverse stiffness. An input coupling to the gear system defines an input coupling lateral stiffness with respect to the gear mesh lateral stiffness and an input coupling transverse stiffness with respect to the hear mesh transverse stiffness.
In a further non-limiting embodiment of any of the foregoing examples, the flexible support lateral stiffness is less than the gear mesh lateral stiffness.
In a further non-limiting embodiment of any of the foregoing examples, the flexible support transverse stiffness is less than the gear mesh transverse stiffness.
In a further non-limiting embodiment of any of the foregoing examples, the flexible support lateral stiffness is less than the gear mesh lateral stiffness, and the flexible support transverse stiffness is less than the gear mesh transverse stiffness.
In a further non-limiting embodiment of any of the foregoing examples, the frame defines a frame lateral stiffness and a frame transverse stiffness.



BRIEF DESCRIPTION OF THE DRAWINGS
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:
FIG. 1 is a schematic cross-section of a gas turbine engine;
FIG. 2 is an enlarged cross-section of a section of the gas turbine engine which illustrates a fan drive gear system (FDGS);
FIG. 3 is a schematic view of a flex mount arrangement for one non-limiting embodiment of the FDGS;
FIG. 4 is a schematic view of a flex mount arrangement for another non-limiting embodiment of the FDGS;
FIG. 5 is a schematic view of a flex mount arrangement for another non-limiting embodiment of a star system FDGS; and
FIG. 6 is a schematic view of a flex mount arrangement for another non-limiting embodiment of a planetary system FDGS.
FIG. 7 is a schematic view of a flex mount arrangement for another non-limiting embodiment of a star system FDGS; and
FIG. 8 is a schematic view of a flex mount arrangement for another non-limiting embodiment of a planetary system FDGS.



DETAILED DESCRIPTION
FIG. 1 schematically illustrates a gas turbine engine 20. The gas turbine engine 20 is disclosed herein as a two-spool turbofan that generally incorporates a fan section 22, a compressor section 24, a combustor section 26 and a turbine section 28. Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section 22 drives air along a bypass flowpath while the compressor section 24 drives air along a core flowpath for compression and communication into the combustor section 26 then expansion through the turbine section 28. 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 architecture gas turbine engine and an open rotor (unducted fan) engine.
The engine 20 generally includes a low speed spool 30 and a 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 38A-38C. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided.
The low speed spool 30 generally includes an inner shaft 40 that interconnects a fan 42, a low pressure compressor 44 and a low pressure turbine 46. The inner shaft 40 is connected to the fan 42 through a geared architecture 48 to drive the fan 42 at a lower speed than the low speed spool 30. The high speed spool 32 includes an outer shaft 50 that interconnects a high pressure compressor 52 and high pressure turbine 54. A combustor 56 is arranged between the high pressure compressor 52 and the high pressure turbine 54. The inner shaft 40 and the outer shaft 50 are concentric and rotate about the engine central longitudinal axis A which is collinear with their longitudinal axes.
The core airflow is compressed by the low pressure compressor 44 then the high pressure compressor 52, mixed and burned with fuel in the combustor 56, then expanded over the high pressure turbine 54 and low pressure turbine 46. The turbines 46, 54 rotationally drive the respective low speed spool 30 and high speed spool 32 in response to the expansion of the airflow passing therethrough.
With reference to FIG. 2, the geared architecture 48 generally includes a fan drive gear system (FDGS) 60 driven by the low speed spool 30 (illustrated schematically) through an input coupling 62. The input coupling 62 both transfers torque from the low speed spool 30 to the geared architecture 48 and facilitates the segregation of vibrations and other transients therebetween. In the disclosed non-limiting embodiment, the FDGS 60 may include an epicyclic gear system which may be, for example, a star system or a planet system.
The input coupling 62 may include an interface spline 64 joined, by a gear spline 66, to a sun gear 68 of the FDGS 60. The sun gear 68 is in meshed engagement with multiple planet gears 70, of which the illustrated planet gear 70 is representative. Each planet gear 70 is rotatably mounted in a planet carrier 72 by a respective planet journal bearing 75. Rotary motion of the sun gear 68 urges each planet gear 70 to rotate about a respective longitudinal axis P.
Each planet gear 70 is also in meshed engagement with rotating ring gear 74 that is mechanically connected to a fan shaft 76. Since the planet gears 70 mesh with both the rotating ring gear 74 as well as the rotating sun gear 68, the planet gears 70 rotate about their own axes to drive the ring gear 74 to rotate about engine axis A. The rotation of the ring gear 74 is conveyed to the fan 42 (FIG. 1) through the fan shaft 76 to thereby drive the fan 42 at a lower speed than the low speed spool 30. It should be understood that the described geared architecture 48 is but a single non-limiting embodiment and that various other geared architectures will alternatively benefit herefrom.
With reference to FIG. 3, a flexible support 78 supports the planet carrier 72 to at least partially support the FDGS 60A with respect to the static structure 36 such as a front center body which facilitates the segregation of vibrations and other transients therebetween. It should be understood that various gas turbine engine case structures may alternatively or additionally provide the static structure and flexible support 78. It is to be understood that the term “lateral” as used herein refers to a perpendicular direction with respect to the axis of rotation A and the term “transverse” refers to a pivotal bending movement with respect to the axis of rotation A so as to absorb deflections which may be otherwise applied to the FDGS 60. The static structure 36 may further include a number 1 and 1.5 bearing support static structure 82 which is commonly referred to as a “K-frame” which supports the number 1 and number 1.5 bearing systems 38A, 38B. Notably, the K-frame bearing support defines a lateral stiffness (represented as Kframe in FIG. 3) and a transverse stiffness (represented as KframeBEND in FIG. 3) as the referenced factors in this non-limiting embodiment.
In this disclosed non-limiting embodiment, the lateral stiffness (KFS; KIC) of both the flexible support 78 and the input coupling 62 are each less than about 11% of the lateral stiffness (Kframe). That is, the lateral stiffness of the entire FDGS 60 is controlled by this lateral stiffness relationship. Alternatively, or in addition to this relationship, the transverse stiffness of both the flexible support 78 and the input coupling 62 are each less than about 11% of the transverse stiffness (KframeBEND). That is, the transverse stiffness of the entire FDGS 60 is controlled by this transverse stiffness relationship.
With reference to FIG. 4, another non-limiting embodiment of a FDGS 60B includes a flexible support 78′ that supports a rotationally fixed ring gear 74′. The fan shaft 76′ is driven by the planet carrier′ in the schematically illustrated planet system which otherwise generally follows the star system architecture of FIG. 3.
With reference to FIG. 5, the lateral stiffness relationship within a FDGS 60C itself (for a star system architecture) is schematically represented. The lateral stiffness (KIC) of an input coupling 62, a lateral stiffness (KFS) of a flexible support 78, a lateral stiffness (KRG) of a ring gear 74 and a lateral stiffness (KJB) of a planet journal bearing 75 are controlled with respect to a lateral stiffness (KGM) of a gear mesh within the FDGS 60.
In the disclosed non-limiting embodiment, the stiffness (KGM) may be defined by the gear mesh between the sun gear 68 and the multiple planet gears 70. The lateral stiffness (KGM) within the FDGS 60 is the referenced factor and the static structure 82′ rigidly supports the fan shaft 76. That is, the fan shaft 76 is supported upon bearing systems 38A, 38B which are essentially rigidly supported by the static structure 82′. The lateral stiffness (KJB) may be mechanically defined by, for example, the stiffness within the planet journal bearing 75 and the lateral stiffness (KRG) of the ring gear 74 may be mechanically defined by, for example, the geometry of the ring gear wings 74L, 74R (FIG. 2).
In the disclosed non-limiting embodiment, the lateral stiffness (KRG) of the ring gear 74 is less than about 12% of the lateral stiffness (KGM) of the gear mesh; the lateral stiffness (KFS) of the flexible support 78 is less than about 8% of the lateral stiffness (KGM) of the gear mesh; the lateral stiffness (KJB) of the planet journal bearing 75 is less than or equal to the lateral stiffness (KGM) of the gear mesh; and the lateral stiffness (KIC) of an input coupling 62 is less than about 5% of the lateral stiffness (KGM) of the gear mesh.
With reference to FIG. 6, another non-limiting embodiment of a lateral stiffness relationship within a FDGS 60D itself are schematically illustrated for a planetary gear system architecture, which otherwise generally follows the star system architecture of FIG. 5.
It should be understood that combinations of the above lateral stiffness relationships may be utilized as well. The lateral stiffness of each of structural components may be readily measured as compared to film stiffness and spline stiffness which may be relatively difficult to determine.
By flex mounting to accommodate misalignment of the shafts under design loads, the FDGS design loads have been reduced by more than 17% which reduces overall engine weight. The flex mount facilitates alignment to increase system life and reliability. The lateral flexibility in the flexible support and input coupling allows the FDGS to essentially ‘float’ with the fan shaft during maneuvers. This allows: (a) the torque transmissions in the fan shaft, the input coupling and the flexible support to remain constant during maneuvers; (b) maneuver induced lateral loads in the fan shaft (which may otherwise potentially misalign gears and damage teeth) to be mainly reacted to through the number 1 and 1.5 bearing support K-frame; and (c) both the flexible support and the input coupling to transmit small amounts of lateral loads into the FDGS. The splines, gear tooth stiffness, journal bearings, and ring gear ligaments are specifically designed to minimize gear tooth stress variations during maneuvers. The other connections to the FDGS are flexible mounts (turbine coupling, case flex mount). These mount spring rates have been determined from analysis and proven in rig and flight testing to isolate the gears from engine maneuver loads. In addition, the planet journal bearing spring rate may also be controlled to support system flexibility.
FIG. 7 is similar to FIG. 5 but shows the transverse stiffness relationships within the FDGS 60C (for a star system architecture). The transverse stiffness (KICBEND) of the input coupling 62, a transverse stiffness (KFSBEND) of the flexible support 78, a transverse stiffness (KRGBEND) of the ring gear 74 and a transverse stiffness (KJBBEND) of the planet journal bearing 75 are controlled with respect to a transverse stiffness (KGMBEND) of the gear mesh within the FDGS 60.
In the disclosed non-limiting embodiment, the stiffness (KGMBEND) may be defined by the gear mesh between the sun gear 68 and the multiple planet gears 70. The transverse stiffness(KGMBEND) within the FDGS 60 is the referenced factor and the static structure 82′ rigidly supports the fan shaft 76. That is, the fan shaft 76 is supported upon bearing systems 38A, 38B which are essentially rigidly supported by the static structure 82′. The transverse stiffness (KJBBEND) may be mechanically defined by, for example, the stiffness within the planet journal bearing 75 and the transverse stiffness (KRGBEND) of the ring gear 74 may be mechanically defined by, for example, the geometry of the ring gear wings 74L, 74R (FIG. 2).
In the disclosed non-limiting embodiment, the transverse stiffness (KRGBEND) of the ring gear 74 is less than about 12% of the transverse stiffness (KGMBEND) of the gear mesh; the transverse stiffness (KFSBEND) of the flexible support 78 is less than about 8% of the transverse stiffness (KGMBEND) of the gear mesh; the transverse stiffness (KJBBEND) of the planet journal bearing 75 is less than or equal to the transverse stiffness (KGMBEND) of the gear mesh; and the transverse stiffness (KICBEND) of an input coupling 62 is less than about 5% of the transverse stiffness (KGMBEND) of the gear mesh.
FIG. 8 is similar to FIG. 6 but shows the transverse stiffness relationship within the FDGS 60D for the planetary gear system architecture.
It should be understood that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to the normal operational attitude of the vehicle and should not be considered otherwise limiting.
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.
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 disclosure.
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 disclosure may be practiced other than as specifically described. For that reason the appended claims should be studied to determine true scope and content.

## Claims

1. A gas turbine engine comprising:
a fan shaft;
at least one bearing supporting the fan shaft;
a frame which supports said fan shaft, said frame defines a frame lateral stiffness and a frame transverse stiffness;
a gear system which drives said fan shaft;
a flexible support which at least partially supports said gear system, said flexible support defines a flexible support lateral stiffness with respect to said frame lateral stiffness and a flexible support transverse stiffness with respect to said frame transverse stiffness; and
an input coupling to said gear system, said input coupling defines an input coupling lateral stiffness with respect to said frame lateral stiffness and an input coupling transverse stiffness with respect to said frame transverse stiffness.

2. The gas turbine engine as recited in claim 1, wherein said flexible support lateral stiffness is less than said frame lateral stiffness.

3. The gas turbine engine as recited in claim 1, wherein said flexible support transverse stiffness is less than said frame transverse stiffness.

4. The gas turbine engine recited in claim 1, wherein said flexible support lateral stiffness is less than said frame lateral stiffness, and said flexible support transverse stiffness is less than said frame transverse stiffness.

5. The gas turbine engine as recited in claim 1, wherein said gear system includes a gear mesh that defines a gear mesh lateral stiffness and a gear mesh transverse stiffness.

6. A gas turbine engine comprising:
a fan shaft;
a frame which supports said fan shaft;
a gear system which drives said fan shaft, said gear system includes a gear mesh that defines a gear mesh lateral stiffness and a gear mesh transverse stiffness;
a flexible support which at least partially supports said gear system, said flexible support defines a flexible support lateral stiffness with respect to said gear mesh lateral stiffness and a flexible support transverse stiffness with respect to said gear mesh transverse stiffness; and
an input coupling to said gear system, said input coupling defines an input coupling lateral stiffness with respect to said gear mesh lateral stiffness and an input coupling transverse stiffness with respect to said gear mesh transverse stiffness.

7. The gas turbine engine as recited in claim 6, wherein said flexible support lateral stiffness is less than said gear mesh lateral stiffness.

8. The gas turbine engine as recited in claim 6, wherein said flexible support transverse stiffness is less than said gear mesh transverse stiffness.

9. The gas turbine engine as recited in claim 6, wherein said flexible support lateral stiffness is less than said gear mesh lateral stiffness, and said flexible support transverse stiffness is less than said gear mesh transverse stiffness.

10. The gas turbine engine as recited in claim 6, wherein said frame defines a frame lateral stiffness and a frame transverse stiffness.

11. A gas turbine engine comprising:
a fan shaft;
a frame which supports said fan shaft, said frame defines at least one of a frame lateral stiffness and a frame transverse stiffness;
a plurality of gears which drive said fan shaft;
a flexible support which at least partially supports said plurality of gears, said flexible support defines at least one of a flexible support lateral stiffness with respect to said frame lateral stiffness and a flexible support transverse stiffness with respect to said frame transverse stiffness; and
an input coupling to said plurality of gears, said input coupling defines at least one of an input coupling lateral stiffness with respect to said frame lateral stiffness and an input coupling transverse stiffness with respect to said frame transverse stiffness, and said at least one of a frame lateral stiffness and a frame transverse stiffness is greater than said at least one of a flexible support lateral stiffness and a flexible support transverse stiffness, and also greater than said at least one of an input coupling lateral stiffness and an input coupling transverse stiffness.