BOLT ASSEMBLY

Abstract

Aspects of the disclosure regard a bolt assembly which comprises a bolt extending in a longitudinal direction through a flange connection, the bolt comprising a first portion and a second portion, the first portion comprising a threaded section at a first side of the flange connection and the second portion comprising a head portion at a second side of the flange connection. The bolt assembly further comprises a nut screwed on the threaded section and a spacer arranged between the nut and the flange connection or between the head portion and the flange connection. The bolt is comprised of a titanium alloy.

Claims

1. A bolt assembly comprising: a bolt extending in a longitudinal direction through a flange connection, the bolt comprising a first portion and a second portion, the first portion comprising a threaded section at a first side of the flange connection, the second portion comprising a head portion at a second side of the flange connection; a nut screwed on the threaded section; and a spacer arranged between the nut and the flange connection or between the head portion and the flange connection; wherein the bolt is comprised of a titanium alloy.

2. The bolt assembly of claim 1, wherein the bolt titanium alloy has at least one of a yield strength above 500 MPa, an elastic modulus smaller than 150 GPa, a density smaller than 5 g/cc and an elongation at failure larger than 15 percent.

3. The bolt assembly of claim 1, wherein the bolt titanium alloy has an elongation at failure larger than 15 percent.

4. The bolt assembly of claim 1, wherein in addition the spacer is comprised of a titanium alloy.

5. The bolt assembly of claim 4, wherein the spacer titanium alloy has a larger elasticity than the bolt titanium alloy.

6. The bolt assembly of claim 5, wherein the spacer is comprised of a Ti-3Al-8V-6Cr-4Mo-4Zr titanium alloy.

7. The bolt assembly of claim 1, wherein the spacer is shaped to have a lower stiffness in the longitudinal direction under compression compared to the stiffness of a strictly cylindrical shape.

8. The bolt assembly of claim 7, wherein the spacer comprises a conic shaped lateral surface.

9. The bolt assembly of claim 7, wherein the spacer comprises a conic through hole.

10. The bolt assembly of claim 7, wherein the spacer comprises a lateral surface shaped as a conic hourglass, a curved hourglass or a mirrored curved hourglass.

11. The bolt assembly of claim 10, wherein the spacer comprises a cylindrical or conic through hole.

12. The bolt assembly of claim 7, wherein the spacer comprises a reverse hourglass through hole.

13. The bolt assembly of claim 7, wherein the spacer comprises a bulged out barrel form.

14. The bolt assembly of claim 7, wherein the spacer comprises a lateral surface shaped as a double stepped cylinder having a larger outer diameter and its ends and a smaller outer diameter in a central section.

15. A bolt assembly comprising: a bolt extending in a longitudinal direction through a flange connection, the bolt comprising a first portion and a second portion, the first portion comprising a threaded section at a first side of the flange connection, the second portion comprising a head portion at a second side of the flange connection; a nut screwed on the threaded section; and a spacer arranged between the nut and the flange connection or between the head portion and the flange connection; wherein the spacer is shaped to have a lower stiffness in the longitudinal direction under compression compared to the stiffness of a strictly cylindrical shape.

16. The bolt assembly of claim 16, wherein the spacer comprises a conic shaped lateral surface.

17. The bolt assembly of claim 15, wherein the spacer comprises a conic through hole.

18. The bolt assembly of claim 17, wherein the spacer comprises a lateral surface shaped as a conic hourglass, a curved hourglass or a mirrored curved hourglass and further comprises a cylindrical or conic through hole.

19. The bolt assembly of claim 15, wherein the spacer comprises a reverse hourglass through hole or a bulged out barrel form.

20. The bolt assembly of claim 15, wherein the spacer comprises a lateral surface shaped as a double stepped cylinder having a larger outer diameter and its ends and a smaller outer diameter in a central section.

Description

[0057] The invention will be explained in more detail on the basis of exemplary embodiments with reference to the accompanying drawings in which:

[0058] FIG. 1 is a sectional side view of a gas turbine engine;

[0059] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0060] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

[0061] FIG. 4 is an embodiment of a bolt assembly comprising a bolt and a spacer, wherein the bolt is comprised of a titanium alloy;

[0062] FIG. 5 is a further embodiment of a bolt assembly comprising a bolt and a spacer, wherein the bolt is comprised of a titanium alloy;

[0063] FIGS. 6-14 depict sectional or perspective views of different embodiments of a spacer that may be implemented in the bolt assemblies of FIGS. 4 and 5; and

[0064] FIG. 15 a fan case having a front flange and an aft flange.

[0065] FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclical gearbox 30.

[0066] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclical gearbox 30 is a reduction gearbox.

[0067] An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclical gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to process around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0068] Note that the terms “low pressure turbine” and “low pressure compressor” as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the “low pressure turbine” and “low pressure compressor” referred to herein may alternatively be known as the “intermediate pressure turbine” and “intermediate pressure compressor”. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

[0069] The epicyclical gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclical gearbox 30 generally comprise at least three planet gears 32.

[0070] The epicyclical gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclical gearbox 30 may be used. By way of further example, the epicyclical gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

[0071] It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

[0072] Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

[0073] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0074] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 20, 22 meaning that the flow through the bypass duct 22 has its own nozzle that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

[0075] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

[0076] In the context of the present invention, the design of bolt assemblies used to connect a fan case with adjacent structures is of relevance. The fan case in the context of which the bolt assemblies are implemented may be the fan case of a geared turbofan engine as discussed with respect to FIGS. 1 to 3 or may generally be the fan case of any gas turbine engine. More particularly, a particularly useful application lies with bolt assemblies of fan cases of Civil Small and Medium Engines, which may have a fan diameter in the range between 35 to 55″. However, the principles of the present invention are not dependent on a particular kind of gas turbine engine or flange connection location.

[0077] To better understand the context in which the present invention may be implemented, the general design of a fan case is initially discussed with respect to FIG. 15. FIG. 15 depicts an embodiment of a fan case 4 circumferentially surrounding a fan. The fan case 4 comprises a front end flange 41 at which it is connected to an engine inlet (not shown), wherein the connection is realized by means of a front flange connection. The front flange connection is also referred to as an A1 connection. The fan case 4 further comprises an aft end flange 42 at which it is connected to further structural elements of the gas turbine engine. The connection is by means of an aft flange connection 45 which is also referred to as A3 connection. The aft flange connection 45 comprises a bolt assembly 5, embodiments of which be discussed with respect to FIGS. 1 to 14. A similar bolt assembly may be implemented at the front flange connection. Several liners or panels may be arranged along an inner surface of the fan case 4 (not shown).

[0078] The bolt assembly 5 is configured to withstand an FBO event and maintain the integrity of the bolt assembly 5 in case of an FBO event.

[0079] FIG. 4 depicts an embodiment of a bolt assembly 5 as may be implemented with an A3 connection or an A1 connection of a fan case 4 as depicted in FIG. 15. The bolt assembly 5 comprises a bolt 51, a nut 52 and a spacer 53. The bolt assembly 5 connects an aft flange 42 of a fan case 4 and a front flange 61 of a further structure 6 to provide for a flange connection 45. The further structure 6 may be outlet guide vane mount ring, for example.

[0080] The bolt 51 generally extends in a longitudinal direction 7 through holes 420, 610 in flanges 42, 61. The assembled bolt 51 comprises a first portion 510 located on a first (in FIG. 4 left) side of the flange connection 45. It further comprises a second portion 511 located on a second (in FIG. 4 right) side of the flange connection 45. The first portion 510 comprises a threaded section 512 onto which the nut 52 is screwed. The spacer 53 is arranged and extends between the nut 52 and the flange 42. The second portion 511 comprises a head portion 513 which rests against the flange 61 through a washer 54.

[0081] The spacer 53 has a generally conic shape, increasing in inner diameter and outer diameter in the longitudinal direction 7, thereby having a reduced stiffness and increased flexibility in the longitudinal direction under compression. Other embodiments of the spacer 53 are discussed with respect to FIGS. 6 to 14.

[0082] By applying a torque on nut 52, the bolt assembly 5 is tightened, wherein a force is exercised by head portion 513 and washer 54 against the axial direction on flange 61 and a force is exercised by spacer 53 in the axial direction on flange 42. Accordingly, the spacer 53 is compressed to some extent depending on the applied torque.

[0083] The bolt 51 is comprised of a titanium alloy which has a yield strength above 500 MPa, an elastic modulus smaller than 150 GPa, a density smaller than 5 g/cc, and an elongation at failure larger than 15 percent. The spacer 53 may be comprised of a Ti-3Al-8V-6Cr-4Mo-4Zr titanium alloy, known as Ti Beta-C. Alternatively, the spacer 53 is made out of steel. The nut 52 and the washer 54 may be made out of steel, such as A286.

[0084] Further, the fan case 4 with flange 42 may be made out of a titanium alloy or made out of steel. The first structure 6 with flange 61 may also be made out of a titanium alloy or made out of steel.

[0085] By forming bolt 51 out of a titanium alloy, the bolt 51 is exposed to less load in an FBO event as the elastic modulus of titanium alloys is relatively low compared to bolts made out of steel or superalloys. At the same time, the spacer provides for an increased flexibility due to its shape and/or material (such as Ti Beta-C) such that it can compress more for a given load compared to a strictly cylindrical design. This “springier” spacer design further reduces the load taken by the bolt 51 in case of an FBO event. It further improves the ability of the bolt assembly 5 to return to its original shape after an FBO event and make up for potentially slightly plastically deformed bolts after an FBO event.

[0086] FIG. 5 shows an alternative embodiment of a bolt assembly 5 which also comprises a bolt 71, a nut 52, a spacer 53 and a washer 54. The materials used for these elements are the same as discussed with respect to FIG. 4.

[0087] The difference to the embodiment of FIG. 4 lies in the arrangement of the elements. In FIG. 5, the bolt 71 generally extends in a longitudinal direction through holes 420, 610 in flanges 42, 61. The assembled bolt 71 comprises a first portion 710 located on a first (in FIG. 5 right) side of the flange connection 45. It further comprises a second portion 711 located on a second (in FIG. 5 left) side of the flange connection 45. The first portion 710 comprises a threaded section 712 onto which the nut 52 is screwed. The nut 52 rests against flange 61 through washer 54. The second portion 711 comprises a head portion 713. The spacer 53 is arranged and extends between a face 7131 of the head portion 713 and flange 42.

[0088] In FIG. 5, the spacer 53 has a cylindrical design. However, in other embodiments, a more flexible design such as the design of FIG. 4 is implemented, wherein a more flexible design reduces the load on bolt 51 in case of an FBO event.

[0089] Except for the spacer design, the difference between the embodiments of FIGS. 4 and 5 lies in the arrangement of the spacer 53 between the nut 52 and the flange connection 45 or between the head portion 713 and the flange connection 45. In the context of the present invention, it is pointed out that both designs may be used to implement the particular materials of the bolt assembly 5 as discussed with respect to FIG. 4.

[0090] FIGS. 6 to 14 show embodiments of a further optimized spacer 53 which is given a design which makes it springier in nature compared to a strictly cylindrical design as shown in FIG. 5. The increased flexibility/reduced stiffness in the longitudinal direction is provided for by the shape of the spacer 53. In additional flexibility may be provided for by the discussed titanium alloy materials which have generally a low elastic modulus. The discussed designs allow the spacer to compress more under the same load by modifying the expansion of the spacer 53 perpendicular to direction of loading (which is the longitudinal direction).

[0091] In FIG. 6, a spacer 53a is provided which comprises a conic shaped lateral surface 532. A through hole 531 of the spacer 53a is cylindrical.

[0092] In FIG. 7, a spacer 53b is provided which has a lateral surface 532 which is curved and comprises cylindrical sections. In this embodiment, the through hole 531 is conic. FIG. 7 also includes a perspective depiction of spacer 53b that further includes the nut 52 and the bolt 51 of FIG. 4.

[0093] FIG. 8 is a combination of the embodiments of FIGS. 6 and 7, wherein both the lateral surface 532 and the through hole 531 are conic.

[0094] In FIG. 9, a spacer 53d is provided which comprises a cylindrical through hole 531. The lateral surface 531 is shaped as a double step cylinder having a larger outer diameter at its ends 532-1 and a smaller diameter in a central section 532-2.

[0095] In FIG. 10, a spacer 53e is provided with a lateral surface 532 which is formed as a conic hourglass, wherein a minimal outer diameter 532-3 is formed in the axial center of the spacer 53e. The through hole 531 is formed as a cylindrical through hole.

[0096] In FIG. 11, a spacer 53f is provided which comprises a lateral surface 532 which is formed as a curved hourglass, wherein the through hole 531 is a cylindrical/straight through hole 531-1 or a conic through hole 531-2.

[0097] In FIG. 12, a spacer 53g is provided which comprises a lateral surface 532 which is formed as a mirrored curved hourglass, wherein the through hole 531 is a cylindrical through hole.

[0098] In FIG. 13, a spacer 53h is provided which comprises a through hole 531 formed as a reverse hourglass and thus comprising the minimal inner diameter at the longitudinal ends of the spacer 53h. The lateral surface 532 is cylindrical.

[0099] In FIG. 14, a spacer 53i is provided which comprises a through hole 531 formed as a reverse hourglass and thus comprising the minimal inner diameter at the longitudinal ends of the spacer 53i. The lateral surface 532 is also formed as a reverse hourglass thus comprising the minimal outer diameter also at the longitudinal ends of the spacer 54i. These shapes of the through hole 531 and of the lateral surface 532 together lead to an overall shape of the spacer 53i in the form of a double hourglass or bulged barrel.

[0100] All the above discussed designs provide for a reduced stiffness in compression while avoiding buckling and maintaining elastic spring-back to preserve bolt integrity.

[0101] It should be understood that the above description is intended for illustrative purposes only and is not intended to limit the scope of the present disclosure in any way. Also, those skilled in the art will appreciate that other aspects of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. Various features of the various embodiments disclosed herein can be combined in different combinations to create new embodiments within the scope of the present disclosure. In particular, the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. Any ranges given herein include any and all specific values within the range and any and all sub-ranges within the given range.