Patent Application
20160107276
The application relates generally to gas turbine engines, and more particularly, to a method for centering engine structure for such engines.
Assembly stack-up may affect the concentricity of engine structures such as the concentricity of the bearing housings with respect to the outer case of a gas turbine engine assembly, which could bring the turbine rotors off center relative to stationary components such as turbine shrouds, thereby directly affecting blade tip and secondary air seal clearance, among other things. Complicated steps have been employed, for example in a mid-turbine frame (MTF) assembly procedure, including stand-off measurements, component numbering, difference calculations, etc. in order to control the concentricity of a bearing housing with respect to an outer case of the MTF.
Accordingly, there is a need to provide an improved method for centering turbine engine cases.
In one aspect, the described subject matter provides a method for making an assembly of a gas turbine engine structure, the assembly including at least co-axially positioned annular outer and inner cases interconnected by a plurality of circumferentially spaced load transfer members extending radially between the outer and inner cases, the method comprising: (1) substantially forming the assembly of the gas turbine engine structure on the fixture by positioning the outer case and the inner case co axially with each other on a fixture and affixing a radially-inner end of each of the load transfer members to the inner case while letting a radially-outer end portion of each of the load transfer members extend radially through one of a plurality circumferentially spaced openings defined in the outer case; (2) creating a plurality of commonly machined surfaces each provided by a machined end surface of one of the load transfer members flush with and surrounded by a machined surface of the outer case, by temporarily securing the substantially formed assembly on the fixture and machining the radially-outer end portion of the respective load transfer members exposed through the respective openings of the outer case and machining an area of the outer case surrounding each of the radially-outer ends; and then (3) securing the co-axial position of the outer and inner cases before the assembly is removed from the fixture by attaching a retaining device to the outer case to retain the respective commonly machined surfaces in place.
In another aspect, the described subject matter provides a method for centering a bearing housing during a mid-turbine frame (MTF) assembly procedure, the method comprising: (1) forming a substantial MTF assembly on the fixture by positioning at least an outer case, an inner case, a plurality of radial spokes and the bearing housing on a fixture, attaching the bearing housing to and supporting the bearing housing in the inner case, affixing a radially-inner end of each of the radial spokes to the inner case while letting a radially-outer end portion of each of the spokes extend radially through one of a plurality circumferentially spaced openings defined in the outer case to expose an outer end of the radially-outer end portion through the respective openings, and positioning the outer case co-axially with the bearing housing; (2) creating a plurality of commonly machined surfaces each formed with a machined end surface of one of the radial spokes flush with and surrounded by a machined surface of the outer case by temporarily securing the substantial MTF assembly on the fixture and machining the radially-outer end of the respective radial spokes exposed through the respective openings of the outer case and machining an area of the outer case surrounding each of the radially-outer ends; and (3) securing the co-axial position of the outer case and the bearing housing to form the MTF for installation in a gas turbine engine by attaching a retaining device to the outer case to retain the respective commonly machined surfaces in place when the substantial MTF assembly is on the fixture.
Further details of these and other aspects of the described subject matter will be apparent from the detailed description and drawings included below.
Reference is now made to the accompanying drawings in which:
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.
Referring to
The terms “axial”, “radial” and “circumferential” used for various components below are defined with respect to the main engine axis shown but not numbered in
Referring to
The MTF 28 may be further provided with an inter-turbine duct (ITD) 110 positioned radially between the outer and inner cases 30, 34, for directing combustion gases to flow through the MTF 28. The ITD 110 may include, for example, an annular duct 112 which has an annular outer duct wall 114 and an annular inner duct wall 116. An annular path 136 is defined between the outer and inner duct walls 114, 116 to direct the combustion gas flow.
The annular duct 112 may further include a plurality of circumferentially-spaced and radially-extending hollow struts 118 (at least three struts) interconnecting the outer and inner duct walls. A plurality of openings 120, 122 may be defined in the respective outer and inner duct walls 114, 116 and may be aligned with the respective hollow struts 118 to allow the respective load transfer members 36 to radially extend through the hollow struts 118. The ITD 110 may be supported and retained within the outer case 30.
A fixture which is schematically illustrated and indicated by numeral 32 (see
Referring to
The substantial MTF assembly held on the fixture 32 may further include the ITD 110 which is positioned radially between the outer and inner cases 30, 34 such that the spokes 36a extend radially through the respective hollow struts 118 and the plurality of openings 120, 122 defined in the respective outer and inner duct walls 114, 116.
The outer case 30 of the substantial MTF assembly held on the fixture 32, may not be secured to the respective spokes 36a while the bearing housing 50 and the spokes 36a are affixed to the inner case 34 such that the outer case 30 can be adjusted or directly positioned by the fixture 32 in a position coaxial with the bearing housing 50. Such a coaxial relationship between the outer case 30 and the bearing housing 50 may be temporarily secured by the fixture 32.
The detailed steps for formation of the substantial MTF assembly on the fixture may vary according to particular connection features of the respective components. For example, the ITD 110 may be configured with a plurality of circumferential segments to allow the respective segments to be radially inwardly placed in position to form the ITD 110 while allowing the respective radially-outwardly extending spokes 36a (which may or may not already be affixed to the inner case 34) to radially extend through the ITD 110. Such various detailed assembly steps are well known in the industry and will not be exhaustively described herein.
The ITD 110 may be supported and retained within the outer case 30, for example by a retaining ring 46 to retain the engagement between the outer duct wall 114 and the outer case 30.
With the substantial MTF assembly being retained on the fixture 32, the radially-outer end 40 of the radially-outer end portion 42 (the spoke 36a) may be exposed through the respective openings 44 in the outer case 30. The radially-outer end 40 of the spokes 36a may slightly project from an entrance (not numbered) of the respective openings 44, as shown in
In a turning operation or a grinding operation the substantial MTF assembly retained on the fixture 32 may be rotated together with the fixture 32 about a rotational axis (not shown) which may or may not be superposed with the central axis (not shown) of the bearing housing 50 such that the plurality of commonly machined surfaces formed by the machined surfaces 40a and machined surfaces 40b define an annular axial plane about the rotational axis.
According to one embodiment employing such a turning or grinding operation, a retaining ring 48 may be placed around the outer case 30 and an annular inner axial surface (not numbered) of the retaining ring may be positioned to be in contact with all of the commonly machined surfaces formed by each combination of the respective machined surfaces 40a and machined surfaces 40b in order to prevent radial movement between the spokes 36a and the outer case 30, thereby securing the co-axial position of the outer case 30 and the bearing housing 50 to form a completed MTF 28 which can then be removed from the fixture 32 and is ready for installation in the gas turbine engine of
Optionally, the outer case 30 may include a plurality of circumferentially spaced connecting bosses 58 projecting radially outwardly from the outer case 30. The openings 44 defined in the outer case 30 may extend radially through the respective connecting bosses 58. Therefore, a top of each boss 58 defines the entries of one of the openings 44 and defines the area surrounding the radially-outer end portion 42 of each spoke 36a. The bosses 58 and the radially-outer end portion 42 of the respective spokes 36a thereby provide the commonly machined surfaces formed by machined surfaces 40a and 40b after the machining operation.
Optionally, the connecting bosses 58 may each be provided with anti-rotation features such as a radial projection 52 engagable with a slot (not numbered) of the radial retaining ring 48 in order to prevent rotational movement of the radial retaining ring 48 with respect to the outer case 30.
Optionally, an axial retaining ring 54 according to a further embodiment may be provided immediately adjacent one axial side of the radial retaining ring 48 and may be engaged in a section of a circumferential groove 56 defined in the respective connecting bosses 58 to prevent the retaining ring 48 from accidentally slipping out of position during engine operation.
Spokes 36a which have become damaged during engine operation may be replaced and the assembly can be re-machined and then retained with a radial retaining ring having a smaller inner surface diameter than that of the previous radial retaining ring 48. The connecting bosses provide such re-machining possibilities without substantially affecting the outer case configuration.
Referring to
Optionally, prior to machining the substantial MTF assembly in situ (i.e. while the substantial MTF assembly is being retained on the fixture) the bearing housing may be masked thoroughly to prevent debris from getting inside the bearing housing during the machining operation. After the machining operation is completed, the substantial MTF assembly held on the fixture may be pressure washed to remove debris before the retaining ring or separate retaining plates are installed.
The above-described embodiments simplify conventional MTF structures where stand-offs have to be measured and numbered, retaining covers have to be measured, numbered and then after calculating the differences, spacers also have to be ground and then numbered. It should also be noted that MTF structures are positioned in a hot area of gas turbine engines and therefore include cold spokes for load transfer and non-structural configuration of gas path and airfoils. All those configurations contribute to hot and cold radial stack-up which negatively affect the concentricity of the bearing housing within the outer case and the repeatability and stackability of the MTF assemblies. However, the machining of the substantial MTF assembly in situ substantially eliminates these tolerances.
The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the described subject matter. For example, a turbofan gas turbine engine has been taken as an examplary application of the described subject matter, however the described subject matter may also be applicable to other types of gas turbine engines. The method of centering the bearing housing with respect to an outer case of a mid turbine frame may also be applicable for the assembly of gas turbine engine structures which include at least co-axially positioned outer and inner cases connected by a plurality of circumferentially spaced load transfer members extending radially between the outer and inner cases. Such a method could include steps similar to those described with reference to the above described embodiments. Modifications which fall within the scope of the described subject matter will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.
