Method and device for connecting fan rotor to low pressure turbine rotor

A rotor assembly includes a fan rotor shaft coupled to a fan rotor, a low pressure turbine rotor shaft coupled to a low pressure turbine rotor, and a joint device configured to connect the fan rotor shaft to the low pressure turbine rotor shaft, to allow torsion, shear and bending to be transferred between the fan rotor shaft and the low pressure turbine rotor shaft under normal operation, and allow torsion and shear but prevent bending to be transferred between the fan rotor shaft and the low pressure turbine rotor shaft under a fan blade-out event.

FIELD OF THE INVENTION

The present disclosure generally relates to gas turbine engine rotor assemblies, and more specifically to devices and methods for connecting fan rotors to low pressure turbine rotors in gas turbine engine rotor assemblies.

BACKGROUND OF THE INVENTION

Gas turbine engines typically include a rotor assembly, a compressor, and a turbine. The rotor assembly includes a fan that includes an array of fan blades extending radially outward from a rotor shaft. The rotor shaft transfers power and rotary motion from the turbine to the compressor and the fan and is supported longitudinally with a plurality of bearing assemblies. Additionally, the rotor assembly has an axis of rotation that passes through a rotor assembly center of gravity. Known bearing assemblies include rolling elements and a paired race, wherein the rolling elements are supported within the paired race. To maintain rotor critical speed margin, the rotor assembly is supported on three bearing assemblies, one of which is a thrust bearing assembly and a second pair of which are roller bearing assemblies. The thrust bearing assembly supports the rotor shaft and minimizes axial and radial movement of the rotor shaft assembly. The remaining roller bearing assemblies support radial movement of the rotor shaft.

During operation of the engine, a fragment of a fan blade may become separated from the remainder of the blade. Accordingly, a substantial rotary unbalance load may be created within the damaged fan and carried substantially by the fan shaft bearings, the fan bearing supports, and the fan support frames.

To minimize the effects of potentially damaging abnormal imbalance loads, known engines include support components for the fan rotor support system that are sized to provide additional strength for the fan support system. However, increasing the strength of the support components undesirably increases an overall weight of the engine and decreases an overall efficiency of the engine when the engine is operated without substantial rotor imbalances.

Other known engines include a bearing support that includes a mechanically weakened section, or primary fuse, that decouples the fan rotor from the fan support system. During such events, the fan shaft seeks a new center of rotation that approximates that of its unbalanced center for gravity. This fuse section, in combination with a rotor clearance allowance, is referred to as a load reduction device, or LRD. The LRD reduces the rotating dynamic loads to the fan support system.

After the primary fuse fails, the pitching fan rotor often induces a large moment to a next closest bearing. The next closest bearing is known as the number two bearing position. The moment induced to the number two bearing induces high bending and stress loads to the fan rotor locally.

It is desirable to provide a gas turbine engine rotor assembly to address at least one of the above-mentioned situations.

SUMMARY OF THE INVENTION

A rotor assembly includes a fan rotor shaft coupled to a fan rotor, a low pressure turbine rotor shaft coupled to a low pressure turbine rotor, and a joint device configured to connect the fan rotor shaft to the low pressure turbine rotor shaft, to allow torsion, shear and bending to be transferred between the fan rotor shaft and the low pressure turbine rotor shaft under normal operation, and allow torsion and shear but prevent bending to be transferred between the fan rotor shaft and the low pressure turbine rotor shaft under a fan blade-out event.

A method for fabricating a rotor assembly for a gas turbine engine include: coupling a fan rotor shaft to a fan rotor; coupling a low pressure turbine rotor shaft to a low pressure turbine rotor; and connecting the fan rotor shaft to the low pressure turbine rotor shaft through a joint device. The joint device is configured to allow torsion, shear and bending to be transferred between the fan rotor shaft and the low pressure turbine rotor shaft under normal operation, and allow torsion and shear but prevent bending to be transferred between the fan rotor shaft and the low pressure turbine rotor shaft under a blade-out event.

A method for reducing unbalanced bearing loading of a gas turbine engine under a fan blade out (FBO) event is provided, wherein the engine includes a fan rotor shaft coupled to a fan rotor and a low pressure turbine rotor shaft coupled to a low pressure turbine rotor. The method includes: operating the gas turbine engine in a normal mode, with the fan rotor shaft and the low pressure turbine rotor shaft connected in torsion, shear and bending; and disconnecting the fan rotor shaft to the low pressure turbine rotor shaft in bending while maintaining the connection between the fan rotor shaft and the low pressure turbine rotor shaft in torsion and shear, when a blade-out event occurs.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present disclosure relate to a rotor assembly of a gas turbine engine, which uses a special joint device to connect a fan rotor shaft (FWD shaft) that is coupled to a fan rotor, to a mid shaft that is coupled to a low pressure turbine (LPT) rotor. The special joint device is configured to allow torsion, shear and bending to be transferred between the fan rotor shaft and the mid shaft under normal operation, and allow torsion and shear but prevent bending to be transferred between the fan rotor shaft and the mid shaft under a fan blade-out event. As such, the fan rotor is basically disconnected from the LPT rotor in bending under the blade-out event, to allow angular misalignment between the fan rotor and the LPT rotor under the blade-out event. Therefore, the application of the special joint device helps reducing bearing loads under the blade-out event and hence helps reducing the overall weight of structural components. As used herein, the term “fan blade-out event” or “blade-out event” refers to fan blade breaking off within the engine.

FIG. 1is a schematic illustration of a gas turbine engine10including a fan assembly12, a high pressure compressor14, and a combustor16. Engine10also includes a high pressure turbine (HPT)18, a low pressure turbine (LPT)20, and a booster22. Fan assembly12includes an array of fan blades24extending radially outward from a rotor disc26. Engine10has an intake side28and an exhaust side30. In one embodiment, the gas turbine engine is a multi-spool turbofan engine, which includes a low pressure spool connecting rotor disc26to a LPT rotor (not shown inFIG. 1) and a high pressure spool connecting the compressor14to a HPT rotor (not shown inFIG. 1). In one embodiment, the gas turbine engine is a GE90 available from General Electric Company, Cincinnati, Ohio.

In operation, air flows through fan assembly12and compressed air is supplied to high pressure compressor14. The highly compressed air is delivered to combustor16. Airflow (not shown inFIG. 1) from combustor16drives turbines18and20, and turbine20drives fan assembly12.

FIG. 2is a schematic cross-sectional view of a rotor assembly40that may be used with a gas turbine engine, such as engine10shown inFIG. 1. Rotor assembly40includes rotor disc26(shown inFIG. 1) and a rotor shaft42which supports an array of fan blades24(shown inFIG. 1) that extend radially outward from rotor disc26. Rotor shaft42is rotatably secured to a structural support frame44with longitudinally spaced bearing assemblies46and48that support rotor shaft42on support frame44. In one embodiment, bearing assembly48is located in a number two bearing position, aft of number one bearing46, and is a fan thrust bearing.

In an exemplary embodiment, each bearing assembly48includes a paired race50and a rolling element52. Paired race50includes an outer race54and an inner race56radially inward from outer race54. Rolling element52is disposed between inner race56and outer race54. Bearing assembly48is enclosed within a sealed annular compartment58radially bounded by rotor shaft42and support frame44. Rolling element52may be a plurality of elements including, but not limited to, a ball bearing or a roller bearing.

Support frame44includes a recess70defined within a bearing support72and sized to receive outer race54. Outer race54is secured within bearing support72with a spanner nut73such that an outer surface74of outer race54is adjacent an inner surface76of bearing support72. A fastener78secures bearing support72and outer race54within recess70. In one embodiment, bearing support72is radially flexible. A face79of outer race54is contoured and sized to receive rolling element52in rollable contact.

Inner race56includes a face80and an inner surface82. Inner race face80is contoured and sized to receive rolling element52in rollable contact. Inner race56is secured within a recess84within a cone shaft86such that inner race inner surface82is adjacent an outer surface88of recess84. In one embodiment, inner race56is split race mating and rolling element52is a ball bearing. In another embodiment, outer race54is split race mating and rolling element52is a ball bearing.

Cone shaft86extends radially outward from fan rotor shaft42and includes an outer portion90, an inner portion92, and a body94extending therebetween. Recess84extends within cone shaft outer portion90and is sized to receive inner race56. A bearing spanner nut96secures inner race56within cone shaft recess84. Body94provides axial and radial support to bearing assembly48. Cone shaft inner portion92includes an inner surface98. Inner surface98is contoured to fit in slidable contact against a face100of a mounting race102. More specifically, in one embodiment, cone shaft inner portion92and mounting race102are known as a mounting joint103.

Mounting race102reduces static loads to rotor assembly40and dynamic loads to support frame44. In one embodiment, mounting race102is secured to fan rotor shaft42with a pair of spacers104and105, and a spanner nut106. Accordingly, mounting race102rotates simultaneously with rotor shaft42. Fan rotor shaft42includes a recess110sized to receive mounting race102and spacers104and105such that a gap (not shown) exists between an inner face114of spacer104and an inner face116of spacer105. Mounting race face100is a spherical surface. In one embodiment, mounting race102is radially thin and is ovalized elastically to assemble to cone shaft inner portion92.

A plurality of mechanical fuses118extend through mounting race102into cone shaft inner portion92to couple mounting race102to cone shaft inner portion92. More specifically, each fuse118extends from an inner surface120of mounting race102through spherical face100and at least partially into cone shaft inner portion92. In the exemplary embodiment, each mechanical fuse118extends radially outward along a centerline axis of mounting race102.

In the exemplary embodiment, each fuse body126has a cross-sectional profile that is substantially constant through body126. In an alternative embodiment, each fuse body126includes an area (not shown) along a shear plane that has a reduced cross-sectional profile. The shear plane is defined as the plane of fuse118at mounting race face100. The reduced cross section at the shear plane provides radial clearance to avoid fuse edge loading and/or stress concentrations during normal spherical deflections. Shear failure, as described in more detail below, will still be the primary failure mode.

During operation of engine10, an unbalance of engine10may cause high radial forces to be applied to fan assembly12(shown inFIG. 1) and a forward most engine bearing. The high radial forces may cause a primary fuse portion118to fail at an engine number one bearing position. The primary fuse failure allows fan assembly12to rotate about a new axis of rotation, passing through a center of gravity of rotor shaft42and inducing bending loads on rotor shaft42that induce a moment load on bearing assembly48at the number two engine bearing position.

Mechanical fuse118is fabricated from a material that fails at a pre-determined moment load applied to rotor shaft42. Furthermore, the material used to fabricate mechanical fuse118, and the design of mounting joint103enables more accurate predictions of the failure point of mechanical fuse118. After mechanical fuse118fails in shear, mounting race spherical face100allows shaft42to pitch such that a shaft center of rotation (not shown) approaches that of the new rotor center of gravity. The spacers104and105allow for rotor pitching such that rotor42does not contact shaft inner portion92.

The pitch rotation occurs once because an unbalance radial load has no relative rotation to shaft42. Rotor shaft42remains in a singular bent position because the unbalance radial load is in a singular location. As a result, mounting race spherical face100does not oscillate and bearing assembly faces79and80remain flush against bearing assembly rolling element52while rotor shaft42rotates. Accordingly, static bending loads transmitted to rotor assembly40are reduced because no moment load is induced through bearing52after mechanical fuses118fail. Furthermore, because no moment load is carried through surfaces98and100, bearing assembly48retains radial and axial load capability.

Because a moment restraint is released, rotor assembly40is permitted to approach the rotor center of gravity and dynamic loads induced to support frame44are reduced. Furthermore, because spherical mounting face100and rolling element52keep rotor shaft42positioned axially with respect to support frame44, turbine clashing between rotor assembly40and a stator assembly (not shown) is substantially eliminated.

The fan rotor shaft42is connected to a LPT rotor shaft (a mid shaft coupled to a LPT rotor)150via a joint device160for transmitting torsion, shear, and bending during normal operation, and transmitting torsion and shear under a blade-out event. Thus, the fan rotor and the LPT rotor are connected in shear, torsion and bending under normal operation, and only connected in shear and torsion under the fan blade-out event.

In the exemplary embodiment, the joint device160includes a spherical spline joint161. As shown inFIG. 3, the spherical spline joint161is a spherical joint with a plurality of circumferentially-spaced spherical splines (curved splines)163, which can be accommodated in and engage spline grooves164defined in the fan rotor shaft42. As used herein, the term “spherical spline” may refer to a spline curve which lies in a sphere or a sphere-like object. In one embodiment, the spherical splines163are substantially parallel to each other and each extends along a curved direction. The spherical splines163are slidable in the spline grooves164along the extending directions thereof, in order to allow relative bending motion between the fan rotor shaft42and the LPT rotor shaft150connected by the spherical spline joint161under the blade-out event. Allowing relative bending motion between the two shafts under the blade-out event helps reducing the natural frequency of the fan rotor and also helps avoiding low pressure to high pressure spool rub.

In the exemplary embodiment, the joint device160is coupled to an aft portion165of the fan rotor shaft42near an aft end167of the fan rotor shaft42, around the number two bearing position, and the joint device160is radially inward from the cone shaft86, with the aft end167of the fan rotor shaft42sandwiched between the cone shaft86and the joint device160. More specifically, the cone shaft86is coupled to a radially outward side of the aft end167via the mounting race102, and the joint device160is coupled to a radially inward side of the aft end167.

The joint device160may further include one or more mechanical fuses169extending into both the fan rotor shaft42and the spherical spline joint161. During normal operation of engine10, the mechanical fuses169can prevent relative bending motion between the fan rotor shaft42and the LPT rotor shaft150, and such that combined loadings of torsion, shear, and bending are transferred between the fan rotor shaft42and the LPT rotor shaft150through the joint device160. Once fan blade-out occurs, loads applied to the fan assembly significantly increase due to appearing unbalance, which causes the mechanical fuses169to fail. The fuse failure allows said relative bending motion between the two shafts. As such, the spherical spline joint161does not transfer bending from input shaft to output shaft but still transfers torque and shear.

The spherical spline joint161may be replaced with any suitable joints that is capable of transmitting torsion, shear, and bending during normal operation, and transmitting torsion and shear but not bending under a blade-out event. Some non-limiting examples of suitable joints include constant velocity universal joints that allow variation in the angle between the input and output shafts and maintain substantially constant angular velocity (such as Rzeppa joints and Birfield joints) and axial couplings for angular misalignment. All these joints that can achieve load reduction by disconnecting the fan rotor shaft from the low pressure turbine rotor shaft in bending under the blade-out event may be named as load reduction joints.

In some embodiments, the joint device160may further include an auxiliary joint for quickly assembling the fan rotor to the LPT rotor and disassembling the fan rotor from the LPT rotor. The auxiliary joint may be a cylindrical spline joint170, because assembly/disassembly of a fan rotor and a LPT rotor connected via a cylindrical spline joint is easier than assembly/disassembly of a fan rotor and a LPT rotor connected via a spherical spline joint. The cylindrical spline joint170may include a plurality of cylindrical splines171and cylindrical spline grooves172for accommodating and engaging the cylindrical splines171. The cylindrical spline joint170may be configured to allow shear, torsion and bending to be transferred either under normal operation or under the fan blade-out event. In the exemplary embodiment, the cylindrical spline grooves172are defined on an inner face of the spherical spline joint161and the cylindrical splines171are formed on an outer face of the LPT rotor shaft150. In other embodiments, the auxiliary joint may be any other joints via which the fan rotor can be assembled to or disassembled from the LPT rotor more easily than via a spherical spline joint, for example, joints that enable the fan rotor and the LPT rotor to be assembled and disassembled along an axial direction of the engine10.

FIG. 4illustrates an exemplary rotor assembly basically similar to the rotor assembly40, except that the fan rotor shaft42is connected to the LPT rotor shaft150via a joint device260including a constant velocity universal joint261instead of the spherical spline joint161. As shown inFIG. 4, the joint device260further includes a cylindrical spline joint270located forward of the constant velocity universal joint261, as the auxiliary joint. The constant velocity universal joint261is coupled to the LPT rotor shaft150. The cylindrical spline joint270includes cylindrical splines formed on one of the constant velocity universal joint261and the aft portion165of fan rotor shaft42, and cylindrical spline grooves defined on the other of the constant velocity universal joint261and the aft portion165, for accommodating and engaging the cylindrical splines.FIG. 5is a perspective view showing a structure of the constant velocity universal joint261according to one exemplary embodiment. In some embodiments, the constant velocity universal joint261may be replaced with an axial coupling361shown inFIG. 6.

Generally under the fan blade-out event, unbalance loads are higher when fan rotor hits its natural frequency (resonance). Since there is no bending connection between the fan rotor shaft42and the LPT rotor shaft150under the fan blade-out event, the natural frequency of fan rotor can be decreased and hence unbalance loads due to the fan blade-out event also can be reduced.

In order to demonstrate at least some advantages of the rotor assemblies as described above, an exemplary rotor assembly of the present disclosure (proposed example) and a comparative rotor assembly (comparative example) are tested to simulate the normal operation and fan blade-out event. In the proposed example, the fan rotor and the LPT rotor are connected through a spherical spline joint as described above to allow torsion and shear but prevent bending to be transferred therebetween under the fan blade-out event. In the comparative example, the fan rotor and the LPT rotor are connected through a cylindrical spline joint to allow shear, torsion and bending to be transferred under the fan blade-out event.

FIG. 7illustrates a status of the fan rotor shaft (FWD shaft) and a LPT rotor shaft (mid shaft) in either proposed example or comparative example under normal operation. “1R”, “2B” “3RB”,4R″ and “5R” inFIG. 7represent the locations (stations) of the number one to five bearings in the engine. As illustrated, there is no bending on the FWD shaft and mid shaft under normal operation.

FIG. 8illustrates a status of the FWD shaft and the mid shaft in the comparative example under the fan blade-out event, andFIG. 9illustrates a status of the FWD shaft and the mid shaft in the proposed example under the fan blade-out event. As illustrated inFIGS. 8 and 9, the fan rotor in the comparative example has a higher natural frequency because of stiffener fan rotor and because of shaft to shaft rub, and has higher bearing loads at decoupled mode, whereas the fan rotor in the proposed example has a much lower natural frequency because of softer fan rotor and because there is no shaft to shaft rub, and has much lower bearing loads because there is no decoupled mode.

FIG. 10compares two curves of loads (on the number two bearing) for the comparative example and the proposed example, respectively. As shown inFIG. 10, the loads on the number two bearings of the proposed example is much lower and more balanced compared to the comparative example.

FIG. 11compares two curves of fan closure for the comparative example and the proposed example, respectively. As shown inFIG. 11, the fan closure in the proposed example is much more balanced compared to the comparative example.