Mount assembly for accessory gearbox of aircraft engine and associated method of assembly

The relative position and orientation between an auxiliary gearbox and a casing of an aircraft engine can be set including engaging a localizing feature of the accessory gearbox with a localizing feature of the casing; and a load path between auxiliary gearbox can subsequently be defined including securing at least two brackets between the accessory gearbox and casing.

TECHNICAL FIELD

The application relates generally to aircraft engines and, more particularly to techniques for mounting an accessory gearbox to the aircraft casing.

BACKGROUND OF THE ART

Aircraft engines typically have an accessory gearbox (AGB), sometimes alternately referred to as auxiliary gearbox or accessory drive, which can serve for exchanging power from the aircraft engine's core and “accessories”. Accessory gearboxes typically provide for the connection to more than one accessory. The exact selection of accessories can vary from one engine to another, but it is relatively common for the accessories to include one or more fuel pump, oil pump, engine starter, etc, all of which can operate on the basis of power from the engine (except for the starter which rather operates to transfer power into the engine at startup). The accessory gearbox can be located externally from the engine's core to avoid the harsher temperatures therein, and be connected thereto by a radially-extending driveshaft. An AGB typically has more than one accessory ports associated to different functions, and the accessory ports are connected to the driveshaft port via gearing, either directly, or via other accessory ports.

Aircraft engine design factors in many different considerations such as power, fuel efficiency, reliability, production costs, maintenance costs, and weight. The design of the mechanical interface between the AGB and the engine's casing is also the subject of this challenge. Although known mechanical interface schemes were satisfactory to a certain degree, there always remains room for improvement.

SUMMARY

In one aspect, there is provided an aircraft engine comprising: a casing extending along and around an axis, the casing having a first localizing feature and a shaft aperture; an accessory gearbox having a second localizing feature and a driveshaft port, the second localizing feature engaged with the first localizing feature; a driveshaft extending radially relative the axis, through the shaft aperture, the driveshaft engaged with the driveshaft port; and at least two brackets including a first bracket having a proximal end secured to the casing on a first circumferential side and a first axial side relative the first localizing feature through at least one clearance hole, and a distal end having at least one first clearance hole, the first bracket fastened to the accessory gearbox through the at least one first clearance hole, and a second bracket having a proximal end secured to the casing on the first circumferential side and a second axial side relative the first localizing feature through at least one clearance hole, and a distal end having at least one second clearance hole, the second bracket fastened to the accessory gearbox through the at least one second clearance hole.

In another aspect, there is provided a method of mounting an accessory gearbox to a casing of an aircraft engine, the method comprising: setting the relative position and orientation between the auxiliary gearbox and the casing including engaging a localizing feature of the accessory gearbox with a localizing feature of the casing; and, defining a load path between auxiliary gearbox, including securing at least two brackets between the previously positioned and oriented accessory gearbox and casing.

In a further aspect, there is provided a mount assembly for defining a load path between an accessory gearbox and a casing of an aircraft engine which are relatively positioned and oriented relative one another, the mount assembly comprising: at least two brackets each having a unibody construction, including a first bracket having a proximal end securable to the casing and a distal end having at least one first clearance hole configured for fastening the first bracket to the accessory gearbox, and a second bracket having a proximal end securable to the casing and a distal end having at least one second clearance hole configured for fastening the second bracket to the accessory gearbox.

In a further aspect, there is provided an aircraft engine comprising: a casing extending along and around an axis, the casing having a first localizing feature and a shaft aperture; an accessory gearbox having a second localizing feature and a driveshaft port, the second localizing feature engaged with the first localizing feature; a driveshaft extending radially relative the axis, through the shaft aperture, the driveshaft engaged with the driveshaft port, a front and a rear defined in opposite directions from the driveshaft along the axis, a left side and a right side defined in opposite directions from the driveshaft in a lateral orientation transversal to the axis; a first bracket having a proximal end secured to the casing on a first one of the left side and right side and a first one of the front and the rear, and a distal end having at least one first clearance hole, the first bracket fastened to the accessory gearbox through the at least one first clearance hole, and a second bracket having a proximal end secured to the casing on the first one of the left side and right side and a second one of the front and the rear, and a distal end having at least one second clearance hole, the second bracket fastened to the accessory gearbox through the at least one second clearance hole.

DETAILED DESCRIPTION

FIG.1illustrates a gas turbine engine10of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication along a main gas path a fan12through which ambient air is propelled, a compressor section14for pressurizing the air, a combustor16in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases around the engine axis11, and a turbine section18for extracting energy from the combustion gases. The compressor14, fan12and turbine18have rotating components which can be mounted on one or more shafts. The exact design varies from one engine type to another, but it is common for turbofan engines, for instance, to have one high pressure shaft20, connecting a high pressure turbine section to the compressor14, and a low pressure shaft22, sometimes referred to as a power shaft, configured to deliver thrust. In the context of a turbofan engine, the low pressure shaft22can connect a lower pressure turbine section to the fan12(which can be considered a low pressure stage of the compressor). In turboshaft and turboprop engines, to name alternate examples of gas turbine engines, the design of the engine core (compressor section, combustor, turbine) can have many similarities with the illustrated design, but the power shaft leads to blades or a propeller, respectively, located outside the casing. Electric and hybrid aircraft engines can also have one or more shaft associated to corresponding rotors.

Aircraft engines have casings which can include a plurality of non-rotary components, and the exact design thereof can vary from one aircraft engine type to another. In the case of gas turbine engines or hybrid engines, the casing24typically has a radially-inner wall26, relative the main axis11, forming a radially outer delimitation to the main gas path, and can also have a radially outer wall28. In the specific case of turbofan engines, the bypass duct30can be considered to form part of the casing24, and forms a radially-outer delimitation to a bypass flow path extending around the engine core, the radially outer wall28forming here a radially-inner delimitation to the bypass flow path.

Aircraft engines typically have an accessory gearbox (AGB)32which can serve for exchanging power from the aircraft engine's core and “accessories”. Accessory gearboxes32typically provide for the connection to more than one accessory. The exact selection of accessories can vary from one engine to another, but it is relatively common for the accessories to include one or more fuel pump, oil pump, engine starter, etc. The accessory gearbox32can be located externally to the casing24, and somewhat remotely from the engine's core to avoid the harsh temperatures which can be sustained during operation of the higher pressure compressors, combustor16, and turbine sections18. Another motivation in the selection of a location of the AGB32externally to the casing would be presence of available space with access to the rotor(s), this can also affect the selection of the top or bottom position relative to the casing, for instance. The accessory gearbox32can be connected to transfer power to an engine shaft (e.g. shaft20) by a radially-extending driveshaft34. As better seen in the example presented inFIG.2A, the AGB32can have a housing36with more than one accessory ports38a,38cassociated to different accessories, and a driveshaft port38b. The accessory ports38a,38care connected to the driveshaft port38binternally to the housing36via gearing, either directly, or via other accessory ports, such as by a train of spur gears interconnected to each other by idler gears (not shown, known in the art).

Given the presence of gearing, and especially in the case of multiple ports, AGBs32can be relatively heavy components. Their placement, especially when remote to the engine's main axis11, can make them prone to dynamic effects, such as resonant vibration frequency response, in addition to more typical structural considerations. AGBs32typically have complex geometries, prompting their manufacture as a component which is initially separate from the casing24and drive shaft34, to which it is designed to be interfaced to. The design of the mechanical interface between the housing36and the casing24may thus be required to take both dynamic and static effects in consideration.

Turning now toFIG.2A, an example embodiment of a mechanical interface between an AGB32and a casing24will now be described. It will be noted that in this embodiment, the localizing function of the mechanical interface can be partially or fully decoupled from the load path function. More specifically, the localizing function is performed by a combination of a first localizing feature40which is provided as part of the casing24, in association with a shaft aperture42, and a second localizing feature44which is provided as part of the AGB32, in association with the driveshaft port38b. The first localizing feature40and shaft aperture42are better shown onFIG.2C, whereas the second localizing feature44and driveshaft port38bare better shown onFIG.2B.FIG.2Apresents the AGB32and the casing24prior to engagement of the first localizing feature40with the second localizing feature44, the engaged position, wherein the relative position and orientation has been set, being shown inFIGS.2D and2E. In the engaged configuration, the driveshaft34extends through the shaft aperture42and is engaged with the driveshaft port38b.

In this embodiment, the load path function can be fully decoupled from the localizing function, and be performed by a mount assembly46including a plurality of brackets48a,48b,48c,48d. All the brackets48a,48b,48c,48dcan be of a unibody construction, such as by being formed of a single component without any articulations therein. The brackets48a,48b,48c,48dcan each have a first end, which can conveniently be referred to as the proximal end50, secured to the casing24, and a distal end52configured for engagement with the AGB32. A body54of each bracket48a,48b,48c,48dextends between the proximal end50and the distal end52, and the body54can extend to a certain extent radially56and circumferentially58relative the engine axis11.

InFIG.2A, the brackets48a,48b,48c,48dhave their proximal end50loosely secured to the casing24, and the distal end52is left free. The relative orientation and position between the AGB32and the casing24is set here entirely independently from the use of the brackets48a,48b,48c,48d, and the brackets48a,48b,48c,48dcan be devoid of localizing features. For instance, in this embodiment, the first localizing feature40can have a peripheral flange60extending around the driveshaft axis61, in the orientation of a length of the drive shaft34(radial orientation relative the main axis11), and can have a tip62forming a length-wise (radial) delimitation to the peripheral flange60. The second localizing feature44can have a male member64sized for snugly engaging the flange60, and sliding lengthwisely, in the orientation of the driveshaft axis61, (radially) along the peripheral flange60until the tip62thereof abuts against an axial seat66.FIGS.2F and2Gillustrate the engagement of the first localising feature40with the second localizing feature44, in accordance with one embodiment, where a seal, such as an O-ring, is provided between the flange60and the male member64to prevent any leakage therebetween.

The flange60and the outer wall of the male member64can both have a corresponding circular cross-section and form a spigot arrangement. The flange60/seat62/male member64engagement can serve to set the relative position of the AGB32and casing24within the plane of the flange, i.e. a plane normal to the driveshaft axis61, and the length of the flange60in the orientation of the driveshaft axis61can set the relative orientation of the AGB32and casing24around two orthogonal axes70,72located in the plane of the flange60. The engagement between the seat66of the male member64and the tip62can set the relative position between the AGB32and the casing24along the driveshaft axis61. When the male member64can rotate relative the flange60around the driveshaft axis61, clocking features can additionally be provided, such as a hole74and bore76with a pin or bolt, for instance, to lock the relative orientation between the AGB32and the casing24around the driveshaft axis61, thereby fully localizing the AGB relative to the casing24in all 6 degrees of freedom. Indeed, the localizing function can be entirely played by the circular seat, which here provides for circumferential, axial, and in-plane localization. The above details are provided in association with one example only, and it will be understood that several alternate configurations are possible in alternate embodiments.

The relative positioning and relative orientation between the AGB32and the casing24, set with the engagement between the first localizing feature40and the second localizing feature44, can be the subject of a tolerance stackup which can affect the exact relative position of the free distal ends52of the brackets48a,48b,48c,48drelative the corresponding portions of the AGB32. To accommodate this potential assembly tolerance stackup, the distal ends52, proximal ends50, or both the distal ends52and the proximal ends50can be provided with one or more clearance holes78designed to accommodate the shafts of corresponding fasteners80. In the illustrated embodiment, both the distal ends52and the proximal ends50have clearance holes78having a similar design, the proximal end50clearance holes78accommodate vertical and lateral assembly tolerances and the distal end52clearance holes78accommodate the axial and lateral assembly tolerances. To this end, the clearance holes78of the proximal ends can be axially-oriented, whereas the clearance holes78of the distal ends52can be upwardly or radially-oriented, for instance. In other words, the orientation of the clearance holes78of the distal ends52and the clearance holes78of the proximal ends50can be different, ideally orthogonal to one another, to provide for adapting to assembly tolerance stackup in all orthogonal orientations.

A degree of clearance c of a given hole78is defined by its difference with the diameter of the shaft of the corresponding fastener80. As opposed to close tolerance, or localizing holes, which are designed with minimal play, or even interference with the shaft, clearance holes78will be designed larger than the shaft of the fastener80which they are designed to accommodate. For instance, as presented inFIG.3D, the clearance hole78can have a greater diameter than the diameter of the shaft80, and the clearance c can be defined as the difference between the two taken from a perfectly centered configuration. As varying tolerance stackups will affect the relative positioning between the clearance holes78and the corresponding threaded bores82of the AGB32from one manufactured engine instance to the other, the clearance c provides a corresponding degree of freedom for the corresponding fastener's shaft80. The load path can be completed once the fastener heads84sandwich the portion of the bracket ends surrounding the corresponding clearance holes c against the corresponding portion of the AGB's housing32or case24either directly, or indirectly (e.g. via a washer or other element). The fastener heads84can transfer, by direct contact, any load components aligned to axis61, while the load component aligned to axis70and72is transferred by friction force caused by fasteners preload.

FIGS.3A and3Bshow two example positions in which the fastener shaft80can ultimately engage the clearance hole78, depending on the embodiment's exact dimensions within the assembly tolerance stackup.FIG.3Cshows an example of a clearance hole78′ which is obround, and adapted to provide a greater degree of clearance in one orientation than in a transverse orientation. Such fastener and clearance hole arrangements can be used at the distal ends52, at the proximal ends50or both, and are used in both the distal ends52and the proximal ends50in the embodiment illustrated inFIG.2A.

The brackets can be thinner in the axial orientation than in the radial56and circumferential58orientations, and remain somewhat flexible in the axial orientation while the distal ends52remain free. If the brackets48a,48b,48c,48dare flexible by hand before securing the distal end52to the AGB32, such as by bending one or both the distal ends52of two axially opposite brackets48a,48ctowards or away from the other, the designer's final selection of clearance c can factor in this flexibility. Indeed, in such a case, it can be assumed that even though the clearance c is not sufficient to accommodate the assembly's tolerance stackup in the axial orientation in and of itself, a certain degree of axial misalignment between the clearance hole78and the threaded bore's82axis (which corresponds to the ultimate position of the fastener) can be accounted for by bending the brackets48a,48b,48c,48dwithin the elastic deformation domain. It will be noted that in the specific embodiment presented inFIG.2A, each one of the brackets48a,48b,48c,48dhas two clearance holes78defined therein. In alternate embodiments, some or all of the brackets48a,48b,48c,48dcan have a single clearance hole78, or more than two clearance holes78, to name some examples. The initially loosely located brackets48a,48b,48c,48dtake their final, closely fitting, position only after securing via the fasteners80, at which point the localizing features can become entirely decoupled from the load path from the point of view of rigidity and structure.

The embodiment presented inFIG.2Ashows a configuration where the accessory gearbox32is provided at a top of the casing24, as opposed to embodiments such as presented inFIG.1where the accessory gearbox can be provided instead at a bottom of the casing. Depending on the specificities of given embodiments, the mount assembly46can have two or more brackets48a,48b,48c,48d. In the embodiment illustrated, as perhaps best seen onFIG.4, the mount assembly46includes four brackets48a,48b,48c,48d, a first and a second brackets48a,48b, being provided on a first axial side86(e.g. front or rear) relative the driveshaft aperture42(and the engaged localizing features40,44), and a third and a fourth brackets48c,48dbeing provided on a second axial side88. The first and the third brackets48a,48care provided on a first circumferential (or lateral) side90relative the driveshaft aperture42/engaged localizing features40,44, and the second and the fourth brackets48b,48dare provided on a second circumferential side92. Such an arrangement can allow the brackets48a,48b,48c,48d, once the distal ends52of which are fastened to the accessory gearbox32, to entirely define the load path between the auxiliary gearbox and the casing24, in the sense that the rigidity of the structure formed by the engagement of the localizing features40,44can be insignificant, or otherwise very small compared to the rigidity of the structure formed by the fully secured brackets48a,48b,48c,48d. In alternate embodiments, a similar objective may be achieved by using three brackets instead of four, for instance. In still other embodiments, it may be preferred for the load path to partially extend via the engaged localization features40,44, which can imply fastening or otherwise securing the engaged localization features, or the vicinity of the engaged localization features, to one another, and in such embodiments, it may be preferred to use only two or three brackets for instance.

The way by which the proximal end50of the brackets48a,48b,48c,48dis secured to the casing24can depend on the specifics of the embodiment. In the embodiment shown inFIG.2AandFIG.4, for instance, the casing24has a sequence of three (or more) sections94,96,98which all extend circumferentially around the main axis11, and which all extend along the axis11. These sections94,96,98include a first section94secured to a second section96at a first structural flange arrangement95, and a third section98secured to the second section96at a second structural flange arrangement97. In alternate embodiments, one or both of the structural flange arrangements may be omitted, or formed as part of a given casing section rather than forming a connector between two casing sections, for instance. In the illustrated embodiment, the first and second structural flange arrangement95,97can include annular flanges protruding radially from a corresponding end of a corresponding section and extending circumferentially around the main axis11. The two corresponding flanges can be secured to one another by fasteners or the like. The structural flange arrangements95,97can provide a greater rigidity of structure than a cylindrical wall portion of the casing24which extends between the flange arrangements95,97. This additional structure can be harnessed in securing the AGB32. In this example, the second section96is the one to which the AGB32is mounted, and which bears both the localizing feature40and the shaft aperture42.

The exact shape and configuration of the brackets48a,48b,48c,48dcan change from one bracket to another, and also from one embodiment to another, and can be affected by the manufacturing technique which is ultimately retained by the designer. In the embodiment ofFIG.2A, for example, the brackets48a,48b,48c,48dare made of folded sheet metal where more specifically, the distal end52is folded into an axially-oriented flange through which the clearance hole(s)78are defined. The proximal end50can extend unfolded and be sandwiched with the corresponding structural flange arrangement95,97via fasteners or the like. In other embodiments, it can be preferred to cast, 3D print, machine from solid, or fabricate the brackets, for instance. Perhaps especially in a sheet-metal construction configuration such as presented inFIG.2A, the body54of the brackets48a,48b,48c,48dwhich extends between the proximal end50and the distal end52can extend to a significantly greater extent radially56and circumferentially58than axially11. This can result in brackets48a,48b,48c,48dwhich are significantly more resistant to stresses occurring in the radial56or circumferential58orientations than to stresses occurring in the axial orientation11, which may be suitable for some embodiments. In the embodiment illustrated, it was preferred to provide a structural web99extending axially between two of the brackets48b,48dwhich are axially aligned with one another, and to secure the structural web99to both these brackets48b,48d. This was found to provide significantly greater resistance of the resulting mount assembly46to stresses exhibited in the axial orientation11, especially when the web99further extends to a certain extent in the radial orientation56. It was found possible to tune the dynamic frequency response of the mounting structure46in the illustrated embodiment simply by changing the thickness of the sheet metal used to form some or all of the brackets48a,48b,48c,48dand web99. The web99can be fastened to corresponding ones of the brackets48a,48b,48c,48dvia clearance holes78as well, to accommodate assembly tolerance stackup along the axial orientation, for instance. A single web99was considered sufficient in the illustrated embodiment, though it will be understood that in alternate embodiments, structural webs can be used between both the first and third48a,48c, and second and fourth48b,48dbrackets, or entirely omitted. It will be noted that with the proposed mount assembly46configuration, the load path can have a footprint91which can be of a significantly greater surface area than the footprint of the localizing features40,44, which can be suitable from the point of view of addressing static and/or dynamic loads.

Accordingly, using a mechanical interface between the accessory gearbox32and the casing24, including a mounting structure46and localizing features40,44such as presented above, a method100of mounting an accessory gearbox32can be represented as shown inFIG.5, and include a step102of setting the relative position and orientation between the auxiliary gearbox32and the casing24including engaging a localizing feature44of the accessory gearbox32with a localizing feature40of the casing24; and a step104of defining a load path between the auxiliary gearbox32and the casing24, including securing at least two brackets48a,48b,48c,48dbetween the positioned and oriented accessory gearbox32and the casing24. It will be noted that defining the load path can include engaging fasteners through clearance holes78formed in the at least two brackets48a,48b,48c,48d, and tightening said fasteners80into threaded bores82formed in the auxiliary gearbox32. Setting the relative position and orientation can include engaging a circular male feature64of the auxiliary gearbox32into a circular flange60of the casing until a seat66of the circular male feature64engages a tip62of the circular flange60. This can include clocking the auxiliary gearbox32including rotating the male feature64within the circular flange60and locking its circumferential orientation. Defining the load path can further include decoupling the load path from the engaged localizing features40,44. Moreover, defining the load path can include bending a distal end52of one or more of the brackets48a,48b,48c,48din the axial orientation, within the elastic deformation domain. In embodiments where the brackets can be bent in the elastic domain in one or more orientations, clearance c may not be required between the holes78′ and the shafts80′ in that or those orientations.

The embodiments described in this document provide non-limiting examples of possible implementations of the present technology. Upon review of the present disclosure, a person of ordinary skill in the art will recognize that changes may be made to the embodiments described herein without departing from the scope of the present technology. For example, in one embodiment, a circular flange can be used to constrain both transversal and radial alignment, and an additional feature can be used to constrain circumferential rotation within the socket formed by the flange, such as a radially oriented locating hole. These features can form a circular seat which can provide for circumferential, axial, and in-plane localization. Different configurations can be used in alternate embodiments. In one embodiment, the casing section can also be made of sheet metal, but this is optional and can be omitted in other embodiments. In some embodiments, the holes can interface the major plane of the bracket at 90 degrees, and can be provided without any shims or rigging, and the brackets can be monolithic and provided without hinge joints, holes or shims. In some alternate embodiments, the locating features can be pins and mating holes instead of a flange and male member, for instance, and the male and female members can be inversed, i.e. the male feature(s) can be provided as part of the casing and the female feature(s) can be provided as part of the AGB. Various types of fasteners can be used to secure the brackets to the AGB and/or casing in different embodiments, such as studs protruding from the AGB secured by nuts, screws, bolts, etc. Yet further modifications could be implemented by a person of ordinary skill in the art in view of the present disclosure, which modifications would be within the scope of the present technology.