Inlet housing of a turbomachine
The input housing of turbomachines is enhanced by using monolithic, additive-manufactured arms with internal partitions, connected through monolithic gussets and pockets, to reduce damage risk and aerodynamic losses, addressing the challenges of frequency coincidences and flow disruption in existing designs.
Patent Information
- Application Number
- FR2022006803
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2022-07-07
- Publication Date
- 2025-04-11
- Estimated Expiration
- 2042-07-07
AI Technical Summary
The input housing of turbomachines is prone to damage due to alternate aerodynamic efforts and vibrations, and the internal partitions used for stiffening do not completely eliminate frequency coincidences, leading to potential damage. Additionally, the current design disrupts the air flow entering the compressor.
The proposed solution involves an input housing with radially extending arms that have a monolithic external envelope and internal partition, manufactured using additive manufacturing to ensure a homogeneous structure and reduce tangential stiffness changes. The arms are connected to the internal hub and external ferrule through monolithic gussets and pockets, which are designed to minimize flow disruption and load losses.
This design reduces the risk of damage to the input housing by minimizing frequency coincidences and enhancing structural integrity, while also reducing aerodynamic load losses by optimizing the flow path through the turbomachine.
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Abstract
Description
Title of the invention: Inlet casing of a turbomachine FIELD OF THE INVENTION
[0001] The present application relates generally to the field of turbomachines, and more particularly to the inlet casing of turbomachines. STATE OF THE ART
[0002] A turbomachine may comprise an inlet casing and a compression section generally comprising a low pressure compressor and a high pressure compressor.
[0003] The role of the inlet casing is to ensure continuity of the flow path at the air inlet sleeve of the turbomachine and to guide the air flow towards the compression section which is located immediately downstream. It also provides a structural function and can, if necessary, support the axial thrust of an engine bearing.
[0004] The inlet casing comprises for this purpose a plurality of arms extending radially between an inner hub and an outer shroud. The arms comprise an aerodynamically profiled outer casing having a pressure side wall, an extrados wall and a trailing edge generally added and fixed downstream of the casing. However, since the inlet casing is placed upstream of the moving wheels of the compression section, the arms are subjected to high mechanical stresses and are subject in particular to alternating aerodynamic forces and significant vibrations due to the operation of the turbomachine. In order to withstand these mechanical stresses, the arms are generally stiffened by internal partitions added and fixed within the casing, between the pressure side wall and the extrados wall. Such partitions make it possible to distance the natural frequencies of the arms from their excitation frequencies and therefore to limit the occurrence of frequency coincidences.However, these partitions do not allow frequency coincidences to be eliminated, so the arms may remain subject to damage during operation.
[0005] In addition, the arms are fixed to the inner hub and the outer shell by means of gussets attached and fixed at the radial ends of the arms. However, since the arms and the gussets are placed in the air flow, they disturb the air flow entering the compressor. Statement of the invention
[0006] An aim of the present application is to remedy the aforementioned drawbacks, by proposing an inlet casing making it possible to reduce the risks of damage to the inlet casing and, where appropriate, the pressure losses in the aerodynamic vein while ensuring its structural function and guiding the air flow.
[0007] For this purpose, according to a first aspect of the invention, there is provided an inlet casing of a turbomachine comprising: - an internal hub extending generally around an axis; - an outer ferrule, extending around the inner hub; - a set of arms extending radially between the inner hub and the outer shell and comprising a closed outer casing delimiting an internal cavity of the arm and at least one partition extending into the internal cavity, the closed casing and the at least one partition being monolithic; the leading edge, the trailing edge, the intrados wall, the extrados wall and the at least one partition being monolithic.
[0008] Some preferred but non-limiting features of the inlet housing according to the first aspect are the following, taken individually or in combination: - the outer envelope defines a leading edge, a trailing edge, a lower surface wall and an upper surface wall, the at least one partition extending between the lower surface wall and the upper surface wall while being fixed to the lower surface wall at a first connection zone and to the upper surface wall at a second connection zone; - a thickness of the intrados wall and the extrados wall is substantially constant at a distance from the first connection zone and the second connection zone; - the at least one partition has a thickness in the center, the intrados wall has an average thickness, the thickness in the center of the at least one partition being equal, to within 10%, to the average thickness of the intrados wall; - wherein the inner hub has an inner surface and an outer surface, the outer ferrule has an inner surface and an outer surface, the arms extending radially between the outer surface of the inner hub and the inner surface of the outer ferrule, each arm having an upper end inserted into a corresponding through slot of the outer ferrule, and the input housing further comprises gussets, each gusset being monolithic with the upper end of a corresponding arm and being fixed on the outer surface of the outer ferrule in order to connect the arm to the outer ferrule; - the outer ferrule further comprises through holes downstream of the upper end of the arms, each through hole being open onto a corresponding through slot; - each arm has a lower end inserted into a corresponding through hole formed in the inner hub, the input housing further comprising monolithic gussets with an inner surface of the inner hub, each gusset of the inner hub being fixed on the lower end of a corresponding arm in order to connect the arm to the inner hub.
[0009] According to a second aspect, the invention proposes a method for manufacturing an inlet casing of a turbomachine according to the first aspect comprising the following steps: - forming by additive manufacturing arms each comprising a leading edge, a trailing edge, a pressure side wall and an extrados wall connecting together the leading edge and the trailing edge and together defining an internal cavity of the arm, and at least one partition extending into the internal cavity between the pressure side wall and the extrados wall, the partition being fixed to the pressure side wall at a first connection zone and to the extrados wall at a second connection zone, so that the leading edge, the trailing edge, the pressure side wall, the extrados wall and the at least one partition are monolithic; - form an internal hub and an external ferrule; and - fix the arms to the internal hub and the external ferrule using gussets.
[0010] Optionally, the manufacturing method further comprises the following steps: - forming through slots in the outer shell; - for each arm, further forming by additive manufacturing the gussets at an upper end of the arms, so that the gussets are monolithic with the upper end of the arms; - insert the arms into the through slots so as to apply the gussets against an external surface of the external shell and fix the gussets on the external surface; - optionally, make through holes in the inner hub, insert a lower end of the arms into a corresponding through hole and fix the lower end of the arms to the inner surface of the inner hub.
[0011] According to a third aspect, the invention also proposes an inlet casing of a turbomachine comprising: - an internal hub extending generally around an axis and having an internal surface and an external surface; - an outer ferrule, extending around the inner hub and having an inner surface and an outer surface; - a set of arms extending radially between the outer surface of the inner hub and the inner surface of the outer ferrule, each arm having an upper end inserted into a corresponding through slot of the outer ferrule; and - gussets, each gusset being monolithic with the upper end of a corresponding arm and being fixed on the external surface of the outer shell in order to connect the arm to the outer shell.
[0012] Some preferred but non-limiting features of the inlet housing according to the third aspect are the following, taken individually or in combination: - the outer ferrule further comprises through holes downstream of the end su upper arms, each through hole being open to a corresponding through slot; and - each arm has a lower end inserted into a corresponding through hole formed in the inner hub, the input housing further comprising monolithic gussets with an inner surface of the inner hub, each gusset of the inner hub being fixed on the lower end of a corresponding arm in order to connect the arm to the inner hub.
[0013] According to a fourth aspect, the invention proposes a turbomachine comprising an inlet casing according to the first and / or the third aspect. DESCRIPTION OF FIGURES
[0014] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which:
[0015] [Fig. 1] is a partial perspective view of an example of an inlet casing according to an embodiment of the invention;
[0016] [Fig.2] is a partial view from below of the inner hub of the input housing of [Fig.l], showing an example of the embodiment of a monolithic gusset with the inner hub;
[0017] [Fig.3] is a partial view of the input housing of [Fig.2] showing the inner hub and outer ferrule;
[0018] [Fig.4] is a sectional view of an exemplary embodiment of an arm of an input casing according to an embodiment of the invention;
[0019] [Fig.5] is a partial view of the input housing of [Fig.l] showing the inner hub, an arm, the outer ferrule and a gusset;
[0020] [Fig. 6] illustrates an example of a turbomachine comprising an inlet casing according to an embodiment of the invention; and
[0021] [Fig.7] illustrates an example of an aircraft which may comprise at least one turbomachine according to one embodiment of the invention.
[0022] Throughout the figures, similar elements bear identical references. DETAILED DESCRIPTION OF THE INVENTION
[0023] A turbomachine 1 (an example of which is illustrated in [Fig.6]) has a main direction extending along a longitudinal axis X ([Fig.3] or indicate the axis X on [Fig.6]) and typically comprises, from upstream to downstream in the direction of gas flow, an inlet casing 2 and a compression section 3 generally comprising a low-pressure compressor and a high-pressure compressor.
[0024] The inlet casing 2 therefore extends upstream of the compression section 3 of the turbomachine 1, immediately downstream of the inlet cone 4 (not referenced in [Fig.6]) of the turbomachine 1.
[0025] In the present application, upstream and downstream are defined relative to the normal flow direction of the gas in the inlet casing 2 and through the turbomachine 1. Furthermore, the X axis of the inlet casing 2 is called its extension X axis, which corresponds substantially to an X axis of symmetry of its internal hub 5 ([Fig.l]?). The axial direction corresponds to the direction of the X axis and a radial direction is a direction perpendicular to this X axis and passing through it. Furthermore, the circumferential (or lateral) direction corresponds to a direction perpendicular to the X axis and not passing through it. Unless otherwise specified, internal (respectively, interior) and external (respectively, exterior), respectively, are used with reference to a radial direction so that the internal part or face of an element is closer to the X axis than the external part or face of the same element.
[0026] The inlet casing 2 ([Fig.l]) comprises an inner hub 5 and an outer shell 6 which are substantially coaxial, as well as arms 7 which extend radially between the inner hub 5 and the outer shell 6. The inner hub 5 extends radially inside the outer shell 6.
[0027] The inner hub 5 ([Fig.2]) comprises an inner surface 8 and an outer surface 9 ([Fig.l]). The outer shell 6 comprises an inner surface 10 ([Fig.3]) and an outer surface 11 ([Fig.l]). The outer surface 9 of the inner hub 5 and the inner surface 10 of the outer shell 6 face each other and are in contact with the gas flow entering the turbomachine 1. The inner surface 8 of the inner hub 5 and the outer surface 11 of the outer shell 6 are not located in the gas flow. The external surface 11 of the external shell 6 faces, for example, the nacelle of the turbomachine 1 and can in particular be positioned opposite a collector 27 ([Fig.l]) of the inlet casing 2. The internal surface 8 of the internal hub 5 can in particular face a set of bearings 29 ([Fig.6]) supporting the drive shaft of the compression section 3.
[0028] In order to reduce the risks of damage to the input casing 2, each arm 7 comprises a closed external casing 30 ([Fig.2]) delimiting an internal cavity 31 and at least one partition 32 extending into the internal cavity 31. In addition, the closed casing 30 and the partition(s) 32 are monolithic. For example, the closed casing 30 and the partition(s) 32 can be obtained by additive manufacturing so as to form a single piece.
[0029] The closed envelope 30 of each arm 7 has an aerodynamic profile suitable for extending into the air flow. It comprises a leading edge 33, a trailing edge 34, a lower surface wall 35 and an upper surface wall 36 of the arm. In a manner known per se, the leading edge 33 is configured to extend opposite the flow of gases entering the inlet casing 2 and then the compression section 3. It corresponds to the front part of an aerodynamic profile which faces the air flow and which divides the airflow into an intrados flow and an extrados flow. The trailing edge 34 corresponds to the rear part of the aerodynamic profile, where the intrados and extrados flows meet.
[0030] All of the elements of the arm 7 (leading edge 33, trailing edge 34, intrados wall 34, extrados wall 35 and partition(s) 32) are therefore monolithic. In this way, the arms 7 do not exhibit any sudden change in tangential stiffness between their lower end 12 ([Fig. 1]) and their upper end 18 ([Fig. 5]), in particular at the level of connection zones 37 ([Fig. 4]) between the partitions 32 and the closed envelope 30 or the trailing edge 34 and the intrados 35 and extrados 36 walls, which limits the formation of cracks under vibratory stresses. The Applicant has in fact noticed that the main damage to the inlet casing 2 was due to the presence of zones of high rigidity transition, these zones corresponding to the mechanical fixing of the partitions 32 and the trailing edge 34 on the intrados 35 and extrados 36 walls of the arms 7.However, by forming the partitions 32, the trailing edge 34 and the intrados 35 and extrados 36 walls integrally and in a single piece, the thickness of the closed envelope 30 can be substantially constant and its variation can be homogeneous and progressive, which eliminates the zones of sudden change in tangential stiffness in the arms 7. This monolithic configuration also makes it possible to homogenize the structure of the arm 7 from a material point of view (no more contribution of different materials for the welding or brazing of the partitions 32 and the trailing edge 34).
[0031] In one embodiment ([Fig.4]), a thickness e2 of the intrados wall 35 and of the extrados wall 36 is substantially constant in the vicinity of the connection zones of the partitions 32 with the closed envelope 30. Thus, each partition 32 has a thickness at the center e2 (i.e. halfway between the intrados wall 35 and the extrados wall 36) and the intrados wall 35 has an average thickness e2 between the lower end 12 and the upper end 18 of the arm 7, measured halfway between the trailing edge 34 and the adjacent partition 32. The thickness e2 at the center of the partition 32 is then equal, to within 10%, to the average thickness e2 of the intrados wall 35. It will be noted that the average thickness e2 of the intrados wall 35 is substantially equal to the average thickness of the extrados wall 36.Thus, the partitions 32 and the intrados 35 and extrados 36 walls have a substantially constant thickness over the height of the arm 7 and from the leading edge 33 to the trailing edge 34. It will be noted that the thickness of the closed envelope 30 increases locally at the level of the connection zone 37 of the partitions 32 and the trailing edge 34 with the intrados wall 35 and the extrados wall 36, insofar as these connection zones 37 each have connection radii in order to facilitate their obtaining by additive manufacturing. However, the connection radius of the connection zones 37 is preferably small to limit the extent of the excess thickness zone. resulting from the connecting rays.
[0032] In one embodiment, each arm 7 comprises two partitions 32 and the trailing edge 34, formed integrally and in a single piece with the closed casing 30. The partitions 32 may extend from the internal hub 5 to the external shell 6, or alternatively over only part of the height of the arm 7. They form an angle of between 75° and 105° with the intrados wall 35 and the extrados wall 36, for example of the order of 90° (to within 2°).
[0033] In order to reduce the pressure losses in the air inlet stream, the upper end 18 of each arm 7 is inserted into a corresponding through slot 19 (visible for example in [Fig.3]) of the outer shell 6. In addition, the inlet casing 2 comprises gussets 17 (see [Fig.5]), each gusset 17 being monolithic with the upper end 18 of a corresponding arm 7 and being fixed on the external surface 11 of the outer shell 6 in order to connect the arm 7 to the outer shell 6.
[0034] The gussets 17 located in the upper part of the arms 7 are therefore placed outside the flow path passing through the inlet casing 2, which makes it possible to eliminate the pressure losses linked to the fixing of the arms 7 on the external shell 6 without adverse impact on the integration, the mechanical strength or the aerothermal functions of the inlet casing 2.
[0035] Preferably, the gussets 17 are fixed by welding or brazing to the external surface 11 of the external shell 6. Each gusset 17 may for example comprise a curved plate extending circumferentially from the upper end 18 of the corresponding arm 7, so as to match the curved shape of the external surface 11 of the external shell 6. The gussets 17 extend in a direction away from the internal cavity 31: the internal cavity 31 of the arms can therefore be open on its upper end 18 because it is not closed by the gussets 17.
[0036] The shape of the slot 19 is complementary to the shape of the upper end 18 of the arm 7 in order to receive the upper end 18 with adjustment and to block the passage of an air flow towards the external surface 11 of the external shell 6 (see [Fig.3]). The slots 19 therefore extend substantially axially.
[0037] If necessary, through holes 25 may further be formed in the outer ferrule 6, downstream of the upper end 18 of the arms 5, in order to allow the installation of the inlet flaps located just behind the arms 7. Each hole 25 is preferably open onto a corresponding slot 19.
[0038] In one embodiment, the gussets 17 are also obtained by additive manufacturing. In this case, the arms 7 can be manufactured from their upper end 18 comprising the gussets 17 towards their lower end 12.
[0039] Where appropriate, the gussets 17 comprise axial grooves, on either side of the upper end 18 of the arms 7. The axial grooves serve to separate the part which will be welded or brazed on the ferrule 6 from the connecting radius between the gusset 17 and the rest of the arm 7. This makes it possible to protect the connecting radius from the welding or brazing operation.
[0040] In the lower part, the lower end 12 of each arm 7 can be inserted into a corresponding through-hole 13 ([Fig.3]) formed in the internal hub 5. In addition, the input casing 2 comprises additional gussets 14 ([Fig.6]), each additional gusset 14 being monolithic with the internal surface 8 of the internal hub 5 and being fixed on the first end 12 of the corresponding arm 7, for example by welding or brazing.
[0041] The additional gussets 14 located in the lower part of the arms 7 are therefore also placed outside the flow path passing through the inlet casing 2, which makes it possible to eliminate the pressure losses linked to the fixing of the arms 7 on the internal hub 5 without adverse impact on the integration, the mechanical strength or the aerothermal functions of the inlet casing 2.
[0042] The shape of the through-orifice 13 is complementary to the shape of the lower end 12 of the arm 7 in order to receive the lower end 12 with adjustment and to block the passage of air from the flow vein towards the internal surface 8 of the internal hub 5. The through-orifices 13 therefore extend substantially axially. Furthermore, as illustrated in [Fig. 2], the gussets 14 comprise a wall 15 extending radially inward from the inner surface 8 of the inner hub 5. The wall 15 of each gusset 14 extends radially in line with the through-hole 13 so that, when the lower end 12 of the arm 7 is inserted into the through-hole 13, the leading edge 33, the trailing edge 34, the intrados wall 35 and the extrados wall 36 of the arm 7 come into contact with the wall 15 of the corresponding gusset 14.The gusset 14 may in particular comprise a protuberance 16, extending from a downstream edge of the through-orifice 13 and configured to come into contact with the trailing edge 34 of the arm 7. The shape of the protuberance 16 is complementary to the shape of the trailing edge 34 of the arm 7 in order to ensure this contact. The shape of the wall 15 is further complementary in the upstream part to the shape of the leading edge 33 of the arm 7.
[0043] The inlet casing 2 may further comprise a collector 27, added and fixed to the external surface 11 of the external shell 6 in line with the arm 7, for example by welding or brazing. The collector 27 may in particular comprise an annular gutter having an upstream annular radial flange configured to be fixed to an upstream edge 28 of the external shell 6, an annular wall substantially coaxial with the axis X and extending at a distance from the external surface 11 of the external shell 6, and a downstream annular radial flange configured to be fixed to an additional edge 29 ( [Fig.3]) of the external ferrule 6 extending downstream of the trailing edge 34 of the arm 7. Inlet Housing Assembly Process 2
[0044] An example of an assembly method will now be described.
[0045] The inner hub 5, the arms 7 and the outer ferrule 6 are manufactured separately.
[0046] The closed envelope 30, the partitions 32 and the gussets 17 of the arms 5 are formed inte generally and in a single piece, for example by additive manufacturing, typically by selective laser melting or electron beam melting, so as to obtain a monolithic part integrating gussets 17.
[0047] Through slots 19 and, where appropriate, through holes 25 are formed in the outer shell 6.
[0048] Through-holes 13 are formed in the inner hub 5. Preferably, the number of through-holes 13 formed in the inner hub 5 and through-slots 19 formed in the outer shell 6 are equal to the number of arms 7 of the inlet casing 2.
[0049] Furthermore, the internal hub 5 is formed integrally and in a single piece with the additional gussets 14, around the through holes 13. The additional gussets 14 are therefore monolithic with the internal hub 5.
[0050] The inner hub 5 and the outer ferrule 6 are placed in a tool in order to position the inner hub 5 radially inside the outer ferrule 6 so that each through hole 13 is aligned with a through slot 19.
[0051] The arms 7 are then inserted radially through the through slots 19 until their lower end 12 enters a corresponding through orifice 13 and their gusset 17 comes into abutment against the external surface 11 of the external shell 6. The lower end 12 of the arms is then fixed to the additional gussets 14 and then the gussets 17 are fixed to the external shell 6, for example by welding or brazing.
[0052] A collector 27 can then be added and fixed to the external ferrule 6.
[0053] The inlet casing 2 thus obtained can then be attached and fixed in a tur- machine 1, in particular of aircraft 100, for example by bolting the downstream edge 26 of the additional shell 22 onto an edge of the compressor casing of the compression section.
Claims
Claims
1. Inlet casing (2) of a turbomachine (1) comprising: - an internal hub (5) extending generally around an axis (X); - an external shroud (6), extending around the internal hub (5); - an arm assembly (7) extending radially between the inner hub (5) and the outer shell (6) and comprising a closed outer casing (30) delimiting an inner cavity (31) of the arm (7) and at least one partition (32) extending into the inner cavity (31), the outer casing (30) and the at least one partition (32) being monolithic, the outer casing (30) defining a leading edge (33), a trailing edge (34), a pressure side wall (35) and an extrados wall (36); the leading edge (33), the trailing edge (34), the pressure side wall (35), the extrados wall (36) and the at least one partition (32) being monolithic;the inner hub (5) having an inner surface (8) and an outer surface (9), the outer ferrule (6) having an inner surface (10) and an outer surface (11), the arms (7) extending radially between the outer surface (9) of the inner hub (5) and the inner surface (10) of the outer ferrule (6); the inlet casing (2) being characterized in that each arm (7) has an upper end (18) inserted into a corresponding through slot (19) of the outer ferrule (6), and in that it further comprises gussets (17), each gusset (17) being monolithic with the upper end (18) of a corresponding arm and being fixed on the outer surface (11) of the outer ferrule (6) in order to connect the arm to the outer ferrule (6).;
2. Inlet casing (2) according to claim 1, wherein the at least one partition (32) extends between the intrados wall (35) and the extrados wall (36) while being fixed to the intrados wall (35) at a first connection zone (37) and to the extrados wall (36) at a second connection zone (37).
3. Inlet casing (2) according to claim 2, wherein a thickness (ei) of the intrados wall (35) and the extrados wall (36) is substantially constant at a distance from the first connecting zone (37) and the second connecting zone (37).
4. Inlet casing (2) according to one of claims 1 to 3, in which the at least one partition (32) has a thickness in the center (e2), the intrados wall (35) has an average thickness (e}), the thickness in the center (e2) of the at least one partition (32) being equal, to within 10%, to the average thickness (e;) of the intrados wall (35).
5. Inlet casing (2) according to one of claims 1 to 4, in which the outer ferrule (6) further comprises through holes (25) downstream of the upper end (18) of the arms (7), each through hole (25) being open onto a corresponding through slot (19).
6. An inlet casing (2) according to one of claims 1 to 5, wherein each arm (7) has a lower end (12) inserted into a corresponding through-hole (13) formed in the inner hub (5), the inlet casing (2) further comprising gussets (14) monolithic with an inner surface (8) of the inner hub (5), each gusset (14) of the inner hub (5) being fixed to the lower end (12) of a corresponding arm (7) in order to connect the arm (7) to the inner hub (5).
7. Turbomachine (1) comprising an inlet casing (2) according to one of claims 1 to 6.
8. A method of manufacturing an inlet casing (2) of a turbomachine (1) according to one of claims 1 to 6 comprising the following steps: - forming by additive manufacturing arms (7) each comprising a leading edge (33), a trailing edge (34), a pressure side wall (35) and an extrados wall (36) connecting together the leading edge (33) and the trailing edge (34) and together defining an internal cavity (31) of the arm (7), and at least one partition (32) extending into the internal cavity (31) between the pressure side wall (35) and the extrados wall (36), the partition (32) being fixed to the pressure side wall (35) at a first connecting zone (37) and to the extrados wall (36) at a second connecting zone (37), so that the leading edge (33), the trailing edge (34), the pressure side wall (35), the extrados wall (36) and the at least one partition (32) are monolithic; - form an internal hub (5) and an external shell (6);and - fix the arms on the internal hub (5) and the external ferrule (6) by means of gussets (14, 17).;
9. A manufacturing method according to claim 8, further comprising the following steps: - forming through slots (19) in the outer ferrule (6); - for each arm (7), further forming by additive manufacturing the gussets (17) at an upper end (18) of the arms, such that the gussets (17) are monolithic with the upper end (18) of the arms (7); - insert the arms (7) into the through slots (19) so as to apply the gussets (14) against an external surface (11) of the external shell (6) and fix the gussets (17) on the external surface (11); - optionally, make through holes (13) in the internal hub (5), insert a lower end (12) of the arms (7) into a corresponding through hole (13) and fix the lower end of the arms (7) on the internal surface (8) of the internal hub (5).