FIR TREE ROOT FOR A BLADED DISC

Abstract

A bladed disc for a turbine engine has disc and blade portions. The disc portion extends in a radial direction from the turbine engine central axis and has slots around its circumference with an inverse fir tree profile. The blade portion has aerofoil and root sections. The root section is configured to have a fir tree profile. The blade portion engages with a slot on the disc portion circumference with the fir tree profile of the root section of the blade portion engaging with the inverse fir tree profile of the slot within the disc portion. The fir tree profile of the root section of the blade portion and the inverse fir tree profile of the slot of the disc portion are curved. The bladed disc has a clearance plate mounted on at least one of the blades, the clearance plate extending across the slot on the disc portion.

Claims

1. A bladed disc for a turbine engine, the bladed disc comprising: a disc portion and a blade portion; the disc portion extends in a radial direction from a central axis of the turbine engine and comprises a plurality of slots around its circumference, the slots being provided with an inverse fir tree profile; and the blade portion comprises an aerofoil section and a root section, the root section being configured to have a fir tree profile; wherein the blade portion engages with a slot on the circumference of the disc portion with the fir tree profile of the root section of the blade portion engaging with the inverse fir tree profile of the slot within the disc portion; the fir tree profile of the root section of the blade portion and the inverse fir tree profile of the slot of the disc portion are curved; and the bladed disc further comprises a clearance plate mounted on at least one of the blades, the clearance plate extending across the slot on the disc portion.

2. The bladed disc as claimed in claim 1, wherein clearance plates are provided on every blade and slot pairing and extend around the disc.

3. The bladed disc as claimed in claim 1, wherein two neighbouring blades are provided with clearance plates and an engagement feature is provided between the two clearance plates to form an interlocking fit.

4. The bladed disc as claimed in claim 1, wherein the fir tree profile of the root section of the blade portion and the inverse fir tree profile of the slot of the disc portion are curved in the radial direction.

5. The bladed disc as claimed in claim 4, wherein the blade portion features a hilt at an interface between the root section and the aerofoil section, wherein the hilt section extends beyond the blade portion along the axis of the engine.

6. The bladed disc as claimed in claim 1, wherein the radial curvature of the fir tree profiles is towards the central axis of the engine.

7. The bladed disc as claimed in claim 1, wherein the radial curvature is convex.

8. The bladed disc as claimed in claim 1, wherein the radial curvature is concave.

9. The bladed disc as claimed in claim 7, wherein the curvature angle is greater than 0 but less than 180.

10. The bladed disc as claimed in claim 9, wherein the curvature angle is 0 to 90.

11. The bladed disc as claimed in claim 1, wherein the fir tree profile on the root section of the blade portion features three projections and the slot on the disc portion features three respective indentations.

12. A gas turbine engine for an aircraft, the gas turbine engine comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades; and a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft, wherein the gas turbine engine incorporates the bladed disc of claim 1.

13. The gas turbine engine as claimed in claim 12, wherein: the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft; the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0050] FIG. 1 is a sectional side view of a gas turbine engine;

[0051] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0052] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

[0053] FIG. 4 is a prior art example of a turbine blade featuring a straight fir tree profile;

[0054] FIG. 5 shows an example of a turbine blade featuring a curved fir tree profile of a blade portion of a bladed disc of the present disclosure;

[0055] FIG. 6 shows a corresponding fir tree profile on the mounting disc portion of a bladed disc of the present disclosure, which is used for mounting the blade portion of FIG. 3;

[0056] FIG. 7 shows the interaction between neighbouring blades of when displacement of one of the blades occurs according to the present disclosure;

[0057] FIG. 8 presents a first embodiment of an additional clearance plate that can be added to the blade to maintain the blade in position;

[0058] FIG. 9 presents a second embodiment of an alternative clearance mechanism featuring a locking clip between them to maintain the blades in position.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0059] Aspects and embodiments of the present disclosure will now be discussed with reference to the accompanying drawings. Further aspects and embodiments will be appreciated by those skilled in the art.

[0060] 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 epicyclic gearbox 30.

[0061] 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 core exhaust 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 epicyclic gearbox 30 is a reduction gearbox.

[0062] 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 epicyclic 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 precess 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.

[0063] 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.

[0064] The epicyclic 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 epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0065] The epicyclic 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 epicyclic gearbox 30 may be used. By way of further example, the epicyclic 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.

[0066] 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.

[0067] 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.

[0068] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0069] 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 18, 20 meaning that the flow through the bypass duct 22 has its own nozzle 18 that is separate to and radially outside the core exhaust 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.

[0070] 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.

[0071] A prior art example of a blade for a compressor or turbine is shown in FIG. 4. The blade is shown having an aerofoil section 401 comprising a pressure surface, a suction surface and a root section 402. The root section has fir tree projections 404 machined into the surface in order to produce a series of engagement features for interlinking with a receiving slot on the disc to which the blade is to be mounted. In this prior art example the projections continue linearly along the width of the root. As discussed above there is a requirement that when the blade is mounted on the disc there is a need for the use of a lock plate, which is required to retain the blade in position at low speeds. This prevents the lateral movement of the blade and the ability of the blade to come off the mounting disc. The presence of this lock plate, however, increases the complexity and the mass of the bladed disc when assembled. As such, the use of lock plates, which are placed between the disc and blades, introduces a high level of complexity to the component which affects the assembly and disassembly procedures. As such the maintenance of the bladed discs becomes much more challenging. Due to their location the lock plates can only be liberated from the rear; this is a process that requires a long disassembly routine. The extraction of the blade currently utilises a destructive process which has the potential to also damage the turbine or compressor disc to which the blades are attached.

[0072] A blade 500 of a compressor or turbine of a turbine engine of the present invention is presented in FIG. 5. In this the blade has the pressure surface 501, suction surface 502 and the root section 503 featuring the fir tree projections 504. However, in this case rather than the projections extending linearly across the width of the root, they instead feature a curve along their length. The curve is in the radial direction, from the centre of the mounting disc to which the blade is mounted, and extends in a plane perpendicular to the plane of the disc. In the example shown in FIG. 5 the curvature is shown to be extending radially outwardly i.e. towards the distal end of a blade. However, the invention is not limited to such an embodiment and the curvature maybe radially inward or towards the proximal end of blade.

[0073] In addition to the blade root featuring a curve, the disc also requires a curve to support the blade for mounting. This is shown in FIG. 6 in which a partial section of the disc 607 demonstrates the curvature of the profile when applied to the disc. The disc has the reverse features 608 to the blade with the profile featuring sections that have been machined away to provide the slots for the projections on the blade to engaged with. These cut away sections continue along the circumference of the disc, such that the blades 605 can be evenly distributed around the disc. The corresponding machining of the blade and disc allows for simple insertion of the blade into the slot within the disc.

[0074] Due to the curved profile of the fir tree any axial movement of the blade will translate to a change in the radial position of the blade. This radial movement will result in interference (or contact) with the neighbouring blades at the edge of their hilts 606, which will act to maintain the blade in position. This is because instead of sliding straight off the disc, the blades instead have to follow the curved path of the fir tree in order to be released. Consequently, this brings them into contact with the neighbouring blade. By bringing one blade into another the blades will, if they have enough momentum, continue to be displaced until they meet a stationary blade on the disc. This is because the motion of the blades will slide into a neighbouring blade around the circumference of the disc. This is therefore a self-locking mechanism in which the differing motion of the blades will ensure that they are retained in position. This offers an improved design that simplifies the disc and blade interaction. The blades and the discs can have the curvature in either direction.

[0075] The number of projections on the fir tree can be any suitable number. This could be one in the case of a dovetail. Similarly, it could for example between 3-5 projections on each blade with the associated indentations on the disc. With respect to the centreline of the disc, the fir tree curvature can be convex or concave with a curvature angle greater than zero degrees and less than or equal to 180 degrees. The curvature of the fir tree corresponds to an arc section. In this, the angle of curvature e represents the arc angle corresponding to an arc length equal to the length curvature of the fir tree on the disc or blade. In certain cases the arc may have a radius equivalent to the disc, however, it is not limited to this. As such, the radius of the circle defining the arc angle can be varied to any suitable value to provide a curve. This angle must be angle greater than zero degrees and less than 180 degrees. In particular the curvature may be 0 to 90. Furthermore, between 20 to 40. The intent of this curve is to oppose the resultant component of centrifugal forces and the axial forces.

[0076] The interference between the displaced blades is shown in FIG. 7 where it can be seen that a hilt 706 at the top of the root section 702 contact with the same part of a neighbouring blade. As the blades are surrounding the disc 707 the effect of the radial displacement is restricted to this region as each of the blades are held in position by each of the neighbouring blades around the disc. The advantage of this design is that it eliminates the need for using of lock plates in the turbine to retain the turbine or compressor blades in place during operation, thus potentially reducing the overall weight of the blade and disc assembly. This design will allow for the blades to be removed from both the front and rear sides of the disc as required. Such a configuration allows for the assembly and disassembly routine to be tailored for the specific disc. If there is no need for the requirements for the blade to be removed from a specific side then a stopper can be incorporated into the disc to ensure that the blades can only be fitted or removed in a particular direction. The improved design thus eliminates the load that lock plates exert on the blades. This removal of the lock plates is beneficial in reducing the wear on the components due to this reduction in strain. The removal of the lock plates also eliminates a potential buckling failure mode for the component, which will improve the reliability of the component.

[0077] As shown in FIG. 7 if the blade starts to move axially a change in the position along the radius from point A towards point B results. This also results in reduction in the horizontal distance from A to C between the blades due to the curvature of the fir tree profiles on the disc and blade. This is because the movement forces the blade to move down a radial line as shown in the figure and which is angled with respect to the neighbouring blade. As such, even if all the blades move in the same direction the continuous radius of the hilts 706 will be reduced, which will result in all of the hilts contacting each other and thus keeping themselves in position. If it is only the movement of a single blade then this will just force it into the neighbouring two blades, which themselves could potentially move. However, this movement will then continue along the adjacent blades, but as they are in a disc they will eventually all come together and lock into position.

[0078] If required a locking mechanism is added to further enhance the security of the blades. This can be added to the blade design across the fir tree and applied to all of the blades on the disc, such that all the other blades are locked in place using this feature. This locking mechanism could be further enhanced by adding features on the blade to control the clearance. The front or rear of the blade is one such location where the additional clearance feature can be placed as shown in FIG. 8. This feature consists of a clearance plate 809 attached to the face of the blade root section 802 and the disc 807 and if attached on one side of the disc will prevent movement of the blade in the direction of the opposite side to the plate. Thus, the blade is prevented form moving by interference with the disc and/or the neighbouring blade or plate. The plate can be formed during the casting or forging of the blades. Alternatively it could be attached or mounted to the blade at a later stage by any suitable connection means; this may include brazing the plate onto the blade. These clearance plates 809 will also allow for tighter clearances for the component as it located further away from the hot zone of the aerofoil in the case of a turbine blade. The benefit of a tighter clearance is that it will reduce the axial motion of the blade. The locking mechanism itself can consist of at least one of these clearance plates 809. These plates may be designed such that when assembled with the blades on the disc they are in close proximity of their neighbour, or may be designed to be slightly greater than the width of the blade, so that the can engage with the disc. Alternatively they may only extend to one side of the blade, and so only engage with the disc at that point. Alternatively, the disc could feature the locking feature. This could be a blanking plate, or feature a projection or a pin to stop the axial movement of the blade. The locking mechanism on the blade and the disc can be used either separately or in conjunction with each other.

[0079] The presence of a locking blade will be required for a complete retention of the blades. This can be done as shown in FIG. 9. Here, clearance plates 909 are added to all of the blades and extend such that they are in close proximity to the neighbouring blades clearance plates; thus having a tighter clearance. This may be to either the front or rear of each of the blades. These clearance plates may include edges that follow along the radial line or can be shaped to produce an interlocking fit. The last blade to be inserted, however, has a smaller clearance plate in order to allow it to be slid in; otherwise it would clash with the clearance plates on the neighbouring blades. Consequently, this clearance plate is configured to have a larger gap in operation between it and the neighbouring clearance plate. Into this gap can be inserted a locking clip 910, which prevents movement of the blades in the axial direction in which the clearance plates are added. This locking clip engages with the two neighbouring clearance plates and locks them together. This locking clip could for example be a T-key bar As such this will ensure that because of the curvature of the blades that if two of them are locked in position that no blades are able to come off the disc when operating at low speeds. As discussed, the locking features may not be just be limited to a single pair of clearance plates but may be distributed around the disc. As the clearance plates are not interlocking even if they are provided with a locking clip they do not have to be removed destructively. As such this can limit the damage caused to the blades and the disc resulting from the removal of the engagement feature over the use of a locking plate. Alternative means of coupling two of the clearance plates together using a locking clip will be apparent to those skilled in the art. For example a fastening or clipping means could be used to ensure the engagement, or they could be engaged by frictional means or by adhesive or physical coupling.

[0080] The blades may be inserted at an angle relative to the plane of the blade. In this instance the root of the blade is angled relative to the hilt and the blade; this means that the blades remain at their standard angle whilst the root to which they are attached are angled. This angling of the root may allow for easier insertion of the blade into the disc. The reason for this is that the hilts are all parallel to each other and minimises interference enough to allow for the assembly of the full ring. The angle of the slot for the in the disc will match the angle of the hilt.

[0081] The curved fir tree can be manufactured with precision electrochemical machining (pECM); however, five-axis milling, electro-discharge machining and additive layer manufacturing methods are potential alternative solutions. Precision electrochemical machining is an electrochemical erosion process utilising oscillating electrodes with a regulated working gap. The process applies a pulsed direct current pulse between the electrode and the workpiece. This workpiece then can dissolve anodically with the geometry of the electrode; this allows for highly complex geometrical shapes to be machined accurately in a repeatable way. As such it is a process that is particularly suited to manufacturing these complex shapes of the curved fir trees on both the blade and the disc. Electric discharge machining is a known machining process in which the fir tree is machined by spark erosion resulting from an electric discharge between a wire and the blade. This process can allow for accurate control when producing the fir tree. 5-axis machining utilises modern computer numerical controls (CNC) to perform this accurate machining of the component. 5-axis machines allow for greater conformity of the final component as either the workpiecethe component to be machinedor the tooling head can be moved along 5 different axes simultaneously. These movement axes are the standard X, Y and Z axis, as well as two rotational axes: the A-axis, which rotates around the X axis; and a C-axis which rotates around the Z-axis. This movement of the workpiece and of the tooling enables the machining of highly complex components such as that of the curved fir trees. Consequently by employing these modern manufacturing techniques allows for the accurate control of the mechanical surfaces which are required for the high tolerance needed to produce these components.

[0082] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.