Load carrying structure

Abstract

A load carrying structure 1 for a gas turbine engine comprises an outer case 5, an inner case 3 and a plurality of struts 2. Each strut comprises first and second ends and extends radially between the

Description

FIELD AND BACKGROUND

[0001] The present invention is concerned with a load carrying structure for a gas turbine engine.

[0002] The operation of a gas turbine engine for example for use in an aircraft is well known to a person skilled in the art who will also be familiar with the arrangement of a conventional turbine mid frame without engine mounts and turbine rear structure without engine mounts, both of which comprise an inner circumferential case portion (a hub portion) and an outer circumferential case portion (a shroud portion).

[0003] A plurality of radially extending struts extend between the inner hub and the outer shroud and provide structural support between the two portions. For a turbine mid frame without engine mounts, a fairing is arranged around each of the struts. Conventionally, the fairing is formed integrally with the spoke frame and the struts / spokes are provided in the form of pretensioned l-rods. In a turbine rear structure with engine mounts, the struts are integrated with the shroud and hub. Pretensioned l-rods are not an option to react engine mount loads.

[0004] The disadvantages with a spoke frame with pretensioned l-rods is that it has limited stiffness, since the cross section of the l-rods is limited by the fairing aerodynamic profile. Resonances in the engine operating range are related to frame stiffness. The l-rod strength is limited by the l-rod pretension required to maintain preload throughout the flight cycle and also restricted by the limitation of the cross sectional area of the l-rod. Given these constraints, the existing load carrying structures are only appropriate for use in turbine mid frames and turbine rear structures without engine mounts (where the required strength capability is not increased).

[0005] Gas turbine engine components experience high levels of pylon drag due to their length. This increases aircraft fuel consumption. The present inventor has established a new and counterintuitive load carrying structure for a gas turbine engine and in particular an arrangement which reduces the turbine rear structure total length by enabling change of the aft engine mount position from the turbine rear structure to the turbine mid frame. This therefore increases the aerodynamic performance of the engine and reduces the specific fuel consumption.

[0006] Gas turbine engines experience high level of rotordynamic loads. The present inventor has established a new and counterintuitive load carrying structure that avoids rotordynamic resonances and enables fulfilment of high cycle fatigue (HCF) requirements.

[0007] Gas turbine engines also experience high core temperatures. The present inventor has established a new and counterintuitive load carrying structure which has a reduced thermal impact thus allowing higher core turbine temperatures.

[0008] An invention described herein is therefore concerned with providing an optimised load carrying structure for a gas turbine engine. SUMMARY

[0009] Particular aspects and embodiments are set out in the appended claims.

[0010] Viewed from a first aspect, there is provided a load carrying structure for a gas turbine engine. The load carrying structure comprises an outer case, an inner case and a plurality of struts. Each of the struts comprises a first end and a second end. The struts extend radially between the outer case and the inner case, wherein the first end of each of the struts comprises an integral stud bolt, wherein the stud bolt extends through an opening in one of the outer case or the inner case.

[0011] The term ‘integral stud bolt’ is intended to mean a part which acts as a bolt and extends from the strut itself. Specifically, the integral stud bolt is not a separate part to the rest of the strut but is formed integrally with it.

[0012] The load carrying structure of the present invention may be used as an aft engine mount for flight engines. The integral stud bolts of the present invention provide increased stiffness which enables them to absorb both mechanical and thermal loads better. This enables moving of the aft engine mounts from the turbine rear structure to the turbine mid structure resulting in a shorter engine. The increased stiffness also enables fulfilment of the HCF requirements by avoiding rotordynamic resonances.

[0013] The load carrying structure of the present invention is improved over existing technologies. The current technology is for an assembled removable load carrying structure with integrally assembled fairing and a spoke frame with pretensioned l-rods / spokes. The disadvantages with a spoke frame with pretensioned l-rods is that is has limited stiffness, since the cross section of the l-rods is limited by the fairing aerodynamic profile. The l-rod strength is limited by the l-rod pretension required to maintain preload throughout the flight cycle. Thus, the l-rod design of existing technologies is not suitable for use in the turbine mid frame where the required strength capability is increased.

[0014] The load carrying structure of the present invention may additionally comprise a fastening means releasably fastened to each of the stud bolts, the fastening means located radially outside the outer case or radially inside the inner case. The fastening means may be a nut. The integral stud bolt may have a threaded outer surface for attaching to the nut. In this way, the nut (or other fastening means) is able to secure the stud bolt to whichever of the inner case or the outer case which it extends through. In other examples, other fastening means may be used in place of the nut.

[0015] The load carrying structure may additionally comprise a locking device releasably attached to each of the fastening means. In this way, the fastening means (such as a nut) can be secured in a closed position to prevent it loosening-. Furthermore, the locking device prevents the bolt preload being relaxed when the part is in operation. The locking device may be a locking wire.

[0016] Each of the integral stud bolts may be preloaded. This is contrary to existing load carrying structures where the tie-rod or strut is preloaded as well since it is designed sufficiently weak and thin to allow preload. In the present invention, the strut is not preloaded as its cross sectional area is maximised within the fairing for increased stiffness. This increased stiffness makes the load carrying structure suitable for use in a turbine mid structure which enables shortening of the engine.

[0017] The second end of each strut comprises a base portion, wherein the base portion is attached to the inner or outer case by one or more bolts. The base portion may have a wider diameter than the rest of the strut. This is in order to receive the bolts for attaching to the base portion to the inner / outer case.

[0018] The second end of each of the struts may additionally comprise an integral stud bolt in place of the base portion wherein the integral stud bolt of the second end extends through an opening in the other of the outer case or the inner case. In this configuration, the strut may not comprise a base portion with a wider diameter. In this configuration therefore, the struts may comprise integral stud bolts at both ends. This may further reduce the weight of the load carrying structure. When an integral stud bolt is used at both ends of the strut, both ends may comprise the fastening means and locking device.

[0019] A maximum thickness of the strut may be larger than a maximum thickness of the integral stud bolt. In other examples, the thickness of the integral stud bolt may be the same as the thickness / width of the strut. The size of the integral stud bolt may be chosen based on mechanical and thermal load. Typically, the integral stud bolt will extend from part of the strut with maximum thickness.

[0020] The end or ends of each strut comprising the integral stud bolt may comprise a face or shoulder portion, wherein the stud bolt extends from the face or shoulder portion.

[0021] On expansion of the strut during use, or at assembly if enabled by small tolerances, the shoulder portion is pressed against the surface of the inner / outer case. This provides increased stiffness of the part, thus making the load carrying structure suitable for use in a turbine mid structure which enables shortening of the engine. Since the strut becomes stiffer with increased cross sectional area, high cycle fatigue requirements can be fulfilled by avoiding resonances with rotordynamic excitations.

[0022] In some examples, the face or shoulder portion may be orthogonal to the radial direction of the strut. Put another way, the face or shoulder portion is parallel to an axial direction of the gas turbine engine.

[0023] The face or shoulder portion may be angled with respect to an axial direction of the gas turbine engine. Specifically, the radial height of one side of the face may be offset with respect to the radial height of the other side of the face. Specifically, the forward edge of the strut may be shorter than the aft edge of the strut. This is because, the outer casing may be formed in a conical shape meaning the offset between the two sides of the strut provides a radial match with the inner surface of the outer casing. This provides an improved frictional contact between the strut and the casing which improves the stiffness of the load carrying structure. In other examples, the aft edge of the strut may be shorter than the forward edge of the strut.

[0024] The face may be substantially flat. In other examples, the face may be curved or may have rounded corners. Specifically, edges of the face may comprise a fillet radius. By having a curved face and / or rounded corners, on expansion of the strut during use, the curved outer face of the strut can be forced against the curved inner face of the outer casing.

[0025] The first and / or second end of each strut may comprise two integral stud bolts. As with a single integral stud bolt, these extend from an end face of the strut. For example, these may both extend through respective openings in the inner and / or outer case. In other examples, there may be more than two integral stud bolts at one or both ends. By having more than one integral stud bolts, the strength capability of the load carrying structure is increased.

[0026] When the load carrying structure is installed within a gas turbine, the struts may be attached to the inner and outer case such that a radially extending gap is formed between the face or shoulder portion of the strut and the inner surface of the outer case and / or the outer surface of the inner case. On expansion of the strut during use, the gap is closed and the shoulder portion is pressed against the surface of the inner / outer case. This radial gap closure results in a joint fixated with friction between the flat face of the stud bolt and the surface of the inner / outer case. This provides increased stiffness of the part, thus making the load carrying structure suitable for use in a turbine mid structure which enables shortening of the engine. Since the strut becomes stiffer with increased cross sectional area, high cycle fatigue requirements can be fulfilled by avoiding resonances with rotordynamic excitations.

[0027] When the load carrying structure is in use, the strut is configured to expand such that the face or shoulder portion comes into contact with the inner surface of the outer case or the outer surface of the inner case. This provides increased stiffness of the part, thus making the load carrying structure suitable for use in a turbine mid structure which enables shortening of the engine. Since the strut becomes stiffer with increased cross sectional area, high cycle fatigue requirements can be fulfilled by avoiding resonances with rotordynamic excitations.

[0028] When the load carrying structure is installed within a gas turbine, the struts may be attached to the inner and outer case such that the face abuts with the inner surface of the outer case and / or the outer surface of the inner case.

[0029] When the part is in use, expansion of the strut increases the stiffness of the load carrying structure as it is forced against the inner surface of the outer case and / or the outer surface of the inner case. If a gap previously existed between the end of the face and the inner surface of the outer case, this is closed by the expansion of the part.

[0030] In some examples, the openings in the outer case and / or the inner case through which the stud bolts extend may be circular in shape. In other examples, these openings may be formed in an obround shape extending in the axial direction of the case. The term ‘obround shape’ is intended to mean a shape consisting of two semicircles connected by parallel lines tangential to their endpoints. In other words, this shape may be described as a stadium or discorectangle shape. The parallel lines of the obround shape extend along the axial direction of the inner / outer case. The width of the obround opening is substantially corresponds to the diameter of the stud bolt.

[0031] For a conical shaped casing or locally conical shaped casing at the strut and casing contact interface, closure of the gap at assembly may be enabled by use of these obround shaped stud bolt holes in the casing. The obround shaped stud bolt holes allow for axial and radial adjustment of the gap between the strut and casing contact surfaces at assembly. Specifically, it is possible to adjust the axial position of the strut at the same time as closing the radial gap between outer case and the strut. The inner or outer case which may be attached to a base portion of the strut may also comprise obround shaped openings for fastening the base portion to the casing.

[0032] The load carrying structure may additionally comprise a plurality of fairings located between the inner and outer case, each fairing encasing a strut. The load carrying structure can be disassembled which enables assembly of a one piece strut fairing. The one piece strut fairing in combination with cooling air reduce the thermal impacton the load carrying structure, and thus allows higher turbine core temperatures. The one piece fairing simplifies assembly of a turbine mid frame or a turbine rear structure with fairing design.

[0033] The plurality of fairings may be formed of a single piece of material. This increases the ease of manufacture. This may be formed by additive manufacturing. This is improved over existing fairings which are cast or made of a sheet of material. By using a one piece fairing, the load carrying structure can be easily disassembled.

[0034] The outer case may be a shroud of a turbine rear structure and wherein the inner case may be a hub of the turbine rear structure.

[0035] The outer case may be a shroud of a mid turbine frame and wherein the inner case may be a hub of the mid turbine frame.

[0036] According to a second aspect, there is provided a method of manufacturing a load carrying structure according to the first aspect of the invention. The method comprises carrying out dimensional stack-up of tolerances, thermal and structural analyses to determine dimensions of the plurality of struts and their respective integral stud bolts.

[0037] Specifically, it is preferred that when the struts and integral stud bolts are joined to the inner and outer case, that the face of the struts abuts with the inner surface of the outer case and / or the outer surface of the inner case. A less preferred but still desirable configuration is that a gap is formed between the face of the struts and the inner surface of the outer case and / or the outer surface of the inner case which gap closes following expansion of the part in use. The use of dimensional stack-up of tolerances, thermal and structural analyses enables these dimensions to be accurately determined.

[0038] The stiffness of the joined parts as well as the fixation of the bolt with preload is therefore qualified by the dimensional stack up of tolerances, thermal and structural analyses to show that the relative displacements of the assembled parts at least results in a closed gap between the joined parts in operating conditions.

[0039] The plurality of struts may be formed by additive manufacturing. This enables the struts with their respective integral stud bolts to be formed accurately to the desired shape and dimensions based on the results of the dimensional stack-up of tolerances, thermal and structural analyses.

[0040] The method may additionally comprise assembling the inner and outer case and the plurality of struts, attaching the integral stud bolt to one of the outer case or the inner case by a fastening means, and locking the fastening means in place using a locking device such as a wire. The fastening means may be a nut. The locking wire ensures that the bolt preload is not relaxed in operating conditions.

[0041] According to a third aspect, there is provided a gas turbine engine comprising the load carrying structure according to the first aspect of the invention. The gas turbine engine may be a flight gas turbine engine suitable for use in aircraft.

[0042] The struts are attached to the inner case and / or outer case through obround openings. Specifically, obround shaped openings may be formed in the inner case and / or outer case which are configured to receive the integral stud bolt or fastenings for attaching to a base portion of a strut.

[0043] Other aspects will also become apparent upon review of the present disclosure, in particular upon review of the Brief Description of the Drawings, Detailed Description and Claims sections. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Examples of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:

[0045] Figure 1 shows a cross-sectional view in a circumferential direction of a strut attached to an inner case of a demonstrator turbine midframe for a gas turbine engine according to the invention;

[0046] Figure 2 shows a perspective view of the strut attached to the inner case and an outer case of the demonstrator turbine midframe;

[0047] Figure 3 shows a schematic cross-sectional view the strut attached to the inner case and the outer case of the demonstrator turbine midframe; and

[0048] Figure 4 shows a cross-sectional view in an axial direction of a fairing and the strut attached to the inner case and the outer case of the demonstrator turbine midframe.

[0049] While the disclosure is susceptible to various modifications and alternative forms, specific example approaches are shown by way of example in the drawings and are herein described in detail. It should be understood however that the drawings and detailed description attached hereto are not intended to limit the disclosure to the particular form disclosed but rather the disclosure is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed invention.

[0050] As used in this specification, the words “comprises”, “comprising”, and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean “including, but not limited to”.

[0051] It will be recognised that the features of the above-described examples of the disclosure can conveniently and interchangeably be used in any suitable combination. It will also be recognised that the invention covers not only individual embodiments but also combinations of the embodiments that have been discussed herein. DETAILED DESCRIPTION

[0052] The present invention is concerned with an improved load carrying structure for a gas turbine engine and a method of manufacturing such a load carrying structure.

[0053] Figure 1 shows a cross-sectional circumferential view of part of the load carrying structure 1 of the present invention. The load carrying structure 1 comprises a strut 2 attached to an inner case 3 and an outer case (not shown in this figure) of a turbine rear structure for a gas turbine engine.

[0054] The strut 2 comprises an integral stud bolt 4 extending from a first end of the strut 2. The first end of the strut comprises a face 11 extending in an orthogonal direction to the radial direction of the strut 2. The combination of the face 11 and the integral stud bolt 4 therefore creates shoulder portions 12 at the first end of the integral stud bolt 4. The integral stud bolt 4 has a smaller cross-sectional area than the rest of the strut. This enables it to be pretensioned.

[0055] The opposing end of the strut comprises a base portion 8. The base portion 8 comprises a greater width than the rest of the strut 2. The base portion abuts the outer surface of the inner case 3 and is attached to the inner case 3 by one or more bolts (not shown in this figure). The base portion 8 is wider than the rest of the strut to receive these bolts. In some examples, openings in the inner case 3 for receiving these bolts may be obround in shape.

[0056] Figure 2 shows a perspective view of the strut 2 of the load carrying structure 1, wherein the strut 2 is attached to the inner case 3 and outer case 5. As can be seen from this figure, the integral stud bolt 4 extends through an opening 9 in the outer case 5. The opening 9 is shown as being circular and sized to receive the integral stud bolt. In other examples, the opening 9 may be obround in shape and may extend along the axial direction of the case 5.

[0057] A nut 6 is attached to the integral stud bolt 4. For example, the outside of the integral stud bolt 4 may be threaded and may be considered to releasably attach to the nut. Although not shown in this figure, a locking wire may be attached to the nut in order to prevent it from opening.

[0058] Figure 3 shows a schematic cross-sectional view of the strut 2 wherein the strut 2 is attached to the inner case 3 and the outer case 5. This view shows the integral stud bolt 4 extending through the opening 9 in the outer case 5. The nut 6 is attached to the integral stud bolt 4 to hold it in place. At the other end, a bolt 10 extends through the inner case 3 and into the base portion 8 of the strut 2. More than one bolt 10 may be used to attach the base portion 8 of the strut 2 to the inner case 3.

[0059] Figure 4 shows a cross-sectional axial view of a fairing 7 fixed in an unstressed state and the strut attached to the inner case 3 and the outer case 5 of the turbine rear structure. The fairing 7 encases the strut 2. As can be seen from this figure, two bolts are added along the axial length of the base portion 8. In other examples, more bolts than this can be used. The fairing is made of a single piece of material by additive manufacturing.

[0060] Although all of these figures show the strut 2 being attached to the inner case 3 by its base portion 8 and one or more bolts 10 and to the outer case 5 by the integral stud bolt 4 and a nut 6, as would be understood by the skilled person the invention is not limited to this. Specifically, rather than extending through the outer case 5, the integral stud bolt 4 may extend through the inner case 3 and the base portion 8 of the strut may abut the inner surface of the outer case 5. Alternatively, the strut 2 may be attached to the inner case 3 and the outer case 5 by integral stud bolts 4. For example, an integral stud bolt 4 may extend from both ends of the strut and through openings in the inner case 3 and the outer case 5. Alternatively, more than one integral stud bolt 4 may extend from one or both ends of the strut 2.

[0061] A method of manufacturing the load carrying structure 1 will now be described. The method comprises carrying out analysis of the dimensional stack-up of tolerances and thermal and structural analyses to determine dimensions of the plurality of struts 2 and their respective integral stud bolts 4.

[0062] Specifically, it is preferred that when the struts 2 and integral stud bolts 4 are joined to the inner and outer case 3, 5, that the face of the struts 2 abuts with the inner surface of the outer case 5 and / or the outer surface of the inner case 3. A less preferred but still desirable configuration is that the gap 11 is formed between the face of the struts 2 and the inner surface of the outer case 5 and / or the outer surface of the inner case 3 which gap 11 closes following expansion of the part in use. The use of analysis of the dimensional stack-up of tolerances and thermal and structural analyses enables these dimensions to be accurately determined.

[0063] The stiffness of the joined parts as well as the fixation of the bolt with preload is therefore qualified by the analysis of the dimensional stack-up of tolerances and thermal and structural analyses to show that the relative displacements of the assembled parts at least results in a closed gap 11 between the joined parts in operating conditions.

[0064] The plurality of struts 2 are formed by additive manufacturing. This enables the struts 2 with their respective integral stud bolts 4 to be formed accurately to the desired shape and dimensions based on the results of the analysis of the dimensional stack-up of tolerances and thermal and structural analyses.

[0065] The method additionally comprises assembling the inner and outer case 3, 5 and the plurality of struts 2, attaching the integral stud bolt 4 to one of the outer case 3 or the inner case 5 by a fastening means such as a nut 6, and locking the fastening means in place using a locking device such as a locking wire.

[0066] The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and / or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the spirit and scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future.

Claims

1. A load carrying structure for a gas turbine engine, the load carrying structure comprising:an outer case;an inner case; anda plurality of struts each comprising a first end and a second end, wherein the struts extend radially between the outer case and the inner case, wherein the first end of each of the struts comprises an integral stud bolt, wherein each stud bolt extends through an opening in one of the outer case or the inner case.

2. The load carrying structure of claim 1, additionally comprising a fastening means releasably fastened to each of the stud bolts, the fastening means located radially outside the outer case or radially inside the inner case.

3. The load carrying structure of claim 2, additionally comprising a locking device releasably attached to each of the fastening means.

4. The load carrying structure of any preceding claim, wherein each of the integral stud bolts is preloaded.

5. The load carrying structure of any one of claims 1 to 4, wherein the second end of each strut comprises a base portion, wherein the base portion is attached to the inner or outer case by one or more bolts.

6. The load carrying structure of any one of claims 1 to 4, wherein the second end of each of the struts additionally comprises an integral stud bolt wherein the integral stud bolt of the second end extends through an opening in the other of the outer case or the inner case.

7. The load carrying structure of any preceding claim, wherein a maximum thickness of the strut is larger than a maximum thickness of the integral stud bolt.

8. The load carrying structure of any preceding claim, wherein the end or ends of each strut comprising the integral stud bolt comprises a face or shoulder portion, wherein the stud bolt extends from the face or shoulder portion.

9. The load carrying structure of claim 8, wherein the face or shoulder portion is orthogonal to the radial direction of the strut.

10. The load carrying structure of claim 8, wherein the face or shoulder portion is angled with respect to an axial direction of the gas turbine engine.

11. The load carrying structure of any of claims 8 to 10, wherein the face or shoulder portion is substantially flat.

12. The load carrying structure of any of claims 8 to 10, wherein the face or shoulder portion is curved.

13. The load carrying structure of any of claims 8 to 12, wherein edges of the face or shoulder portion comprise a fillet radius.

14. The load carrying structure of any preceding claim, wherein the first and / or second end of each strut comprises two integral stud bolts.

15. The load carrying structure of any of claims 8 to 14, wherein when the load carrying structure is installed within a gas turbine, the struts are attached to the inner and outer case such that a radially extending gap is formed between the face and the inner surface of the outer case and / or the outer surface of the inner case.

16. The load carrying structure of any of claims 8 to 14, wherein when the load carrying structure is installed within a gas turbine, the struts are attached to the inner and outer case such that the face abuts with the inner surface of the outer case and / or the outer surface of the inner case.

17. The load carrying structure of claim 15 or claim 16, wherein when the gas turbine engine is in use, the strut is configured to expand such that the face comes into contact with the inner surface of the outer case or the outer surface of the inner case.

18. The load carrying structure of any preceding claim, wherein the load carrying structure additionally comprises a plurality of fairings located between the inner and outer case, each fairing encasing a strut.

19. The load carrying structure of claim 18, wherein the plurality of fairings are formed of a single piece of material.

20. The load carrying structure of any preceding claim wherein the openings in the outer case and / or the inner case through which the stud bolts extend are an obround shape and extend in the axial direction of the case.

21. The load carrying structure of any preceding claim, wherein the outer case is a shroud of a turbine rear structure and wherein the inner case is a hub of the turbine rear structure.

22. The load carrying structure of any preceding claim, wherein the outer case is a shroud of a mid turbine frame and wherein the inner case is a hub of the mid turbine frame.

23. A method of manufacturing a load carrying structure according to any preceding claim, the method comprising:carrying out analyses to determine dimensions of the plurality of struts and their respective integral stud bolts.

24. The method according to claim 22, wherein the analyses comprise analysis of dimensional stack-up of tolerances and thermal and structural analyses.

25. The method according to claim 22 or 23, wherein the plurality of struts are formed by additive manufacturing.

26. The method according to any of claims 22 to 24, additionally comprising forming a fairing for each of the plurality of struts by additive manufacturing.

27. The method according to any of claims 22 to 25, additionally comprising: assembling the inner and outer case and the plurality of struts;attaching the integral stud bolt to one of the outer case or the inner case by a fastening means; andlocking the fastening means in place using a locking device.

28. A gas turbine engine comprising the load carrying structure of any of claims 1 to 22.

29. The gas turbine engine of claim 28, wherein the struts are attached to the inner caseand / or outer case through obround openings.

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