FLEXIBLE SUPPORT STRUCTURE FOR A GEARED
ARCHITECTURE GAS TURBINE ENGINE
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present disclosure is a continuation-in-part of U.S. Patent Application No.
13/342,508, filed January 3, 2012, which claims priority to U.S. Provisional Patent Application
No. 61/494453, filed June 8, 2011.
BACKGROUND
[0002] The present disclosure relates to a gas turbine engine, and more particularly to
a flexible support structure for a geared architecture 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. 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.
[0004] 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.
[0005] 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
[0006] 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.
[0007] In a further non-limiting embodiment, the flexible support lateral stiffness is
less than the frame lateral stiffness.
[0008] In a further non-limiting embodiment of any of the foregoing examples, the
flexible support transverse stiffness is less than the frame transverse stiffness.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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
[0016] 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:
[0017] Figure 1 is a schematic cross-section of a gas turbine engine;
[0018] Figure 2 is an enlarged cross-section of a section of the gas turbine engine
which illustrates a fan drive gear system (FDGS);
[0019] Figure 3 is a schematic view of a flex mount arrangement for one non-limiting
embodiment of the FDGS;
[0020] Figure 4 is a schematic view of a flex mount arrangement for another nonlimiting
embodiment of the FDGS;
[0021] Figure 5 is a schematic view of a flex mount arrangement for another nonlimiting
embodiment of a star system FDGS; and
[0022] Figure 6 is a schematic view of a flex mount arrangement for another nonlimiting
embodiment of a planetary system FDGS.
[0023] Figure 7 is a schematic view of a flex mount arrangement for another nonlimiting
embodiment of a star system FDGS; and
[0024] Figure 8 is a schematic view of a flex mount arrangement for another nonlimiting
embodiment of a planetary system FDGS.
DETAILED DESCRIPTION
[0025] Figure 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] With reference to Figure 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.
[0030] 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.
[0031] 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 (Figure 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.
[0032] With reference to Figure 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 Figure 3) and a transverse stiffness (represented as KframeBEND in
Figure 3) as the referenced factors in this non-limiting embodiment.
[0033] 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 (Kframe END) . That is, the transverse stiffness of the entire FDGS 60 is
controlled by this transverse stiffness relationship.
[0034] With reference to Figure 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 72' in the schematically illustrated planet system which otherwise
generally follows the star system architecture of Figure 3.
[0035] With reference to Figure 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.
[0036] 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 (Figure 2).
[0037] 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.
[0038] With reference to Figure 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 Figure 5.
[0039] 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.
[0040] 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.
[0041] Figure 7 is similar to Figure 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 (KFS END) 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.
[0042] 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 (Figure 2).
[0043] 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.
[0044] Figure 8 is similar to Figure 6 but shows the transverse stiffness relationship
within the FDGS 60D for the planetary gear system architecture.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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
What is claimed is:
1. A gas turbine engine comprising:
a 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 hear 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.