Coupling shaft for an aircraft turbine engine

Abstract

The invention relates to a coupling shaft (8b) for an aircraft turbine engine (1), said shaft (8b) extending along and around a longitudinal axis (A) and having an annular wall (8ba) which comprises a first longitudinal end connected by a first annular web (34) to a first annular flange (38), and a second longitudinal end connected by a second annular web (26) to a second annular flange (28), each of the first and second webs (26, 34) having an inner periphery connected to the annular wall (8ba) and an outer periphery connected to the corresponding flange (28, 38), which extends radially outwards from this outer periphery, characterized in that the first web (34) has a generally frustoconical shape which flares from its inner periphery to its outer periphery towards the second web (26).

Description

[0001]DESCRIPTION TITLE: COUPLING SHAFT FOR AN AIRCRAFT TURBOMACHINE Technical field of the invention The present invention relates to a coupling shaft for an aircraft turbomachine, and to an aircraft turbomachine comprising such a shaft. The coupling is of the flex coupling type. Technical background The state of the art includes in particular documents EP-A1-3153680, US-A1-5,433,674, FR-A1-2979121, FR-A1-3075878 and FR-A1-3075880. A turbomachine, such as a bypass turbojet, conventionally comprises an air inlet comprising a fan whose outlet air flow is divided into an air flow which enters the engine and forms a hot flow or primary flow, and an air flow which flows around the engine and which forms a cold flow or secondary flow. The engine typically comprises from upstream to downstream, in the direction of gas flow, at least one compressor, a combustion chamber, at least one turbine, and an ejection nozzle into which the gasescombustion coming out of the turbine and forming the primary flow are mixed with the secondary flow. A turbomachine can also be of the "double-body" type, which means that it has two rotors arranged coaxially. A first body is called the low-pressure body and a second body is called the high-pressure body. As is known, the engine in this case comprises, from upstream to downstream, a low-pressure compressor, a high-pressure compressor, the combustion chamber, a high-pressure turbine and a low-pressure turbine. In the case of a turbomachine with a reduction gear, the turbine shaft drives the fan shaft via the reduction gear which is lubricated and housed in a lubrication enclosure. Depending on the type of reduction gear used, planetary or epicyclic, the fan shaft will rotate in the same direction or in the opposite direction to the turbine shaft, and the fan shaft will rotate at a lower speed than that of the turbine shaft. The fan shaftturbine, which is the low-pressure turbine shaft in the case of a twin-spool turbomachine, is generally coupled to a low-pressure compressor shaft which is itself coupled to an input shaft of the reducer. This input shaft passes through the reducer and engages the latter's solar. To ensure proper operation of this low-pressure shaft line and in particular of the reducer, it is necessary to transmit the torque but, at the same time, obtain a certain flexibility in the input shaft of the reducer so as not to transmit significant forces in the reducer due to displacements at the imposed interfaces. One solution consists of connecting the shafts of the shaft line by coupling devices which give a certain flexibility to the shaft line in operation. Among the known coupling devices for a turbomachine, we can cite the technologies designated by their English names "flex coupling" and "curvic coupling".The aforementioned documents describe coupling devices of the "flex coupling" type. In this type of device, two shafts are connected together by flanges tightened against each other by means of screws which ensure the transmission of torque between the shafts. The flanges are located at the outer periphery of webs which extend radially outwards from the tubular bodies of the shafts. It is the presence of these webs which makes it possible to give flexibility to the coupling. More specifically, the geometry and dimensions of the webs make it possible to adjust the flexibility of the coupling. In operation, due to the centrifugal forces which are applied to the shaft line, the presence of this type of coupling induces an axial contraction effect of the shaft line. In addition, a mechanical reducer is sensitive to misalignments, in particular axial misalignments, because this generates an additional axial force which the gear teeth must absorb.The two phenomena that contribute to these misalignments are the axial displacements imposed by the engine and the aforementioned axial deformation of the shaft line under centrifugal effect. The greater the stiffness of the shaft line, the greater the axial force because this axial force is proportional to this stiffness. In order to limit this axial force, one solution would therefore be to minimize the axial stiffness of the shaft line or to have an axial deformation of this shaft line that is as close as possible to the displacements imposed by the engine, so that the difference between the two tends towards zero. In other words, the shaft line would have to be able to absorb the displacements imposed by the engine. However, with current flex coupling technology, the axial contraction of the shaft line is added to the displacements imposed by the engine and reducing the axial stiffness of the shaft increases this axial contraction effect. The present invention proposes a simple solution,efficient and economical to the need expressed above. Summary of the invention The invention relates to a coupling shaft for an aircraft turbomachine, this shaft extending along and around a longitudinal axis and comprising an annular wall which comprises a first longitudinal end connected by a first annular web to a first annular flange, and a second longitudinal end connected by a second annular web to a second annular flange, each of the first and second webs comprising an inner periphery connected to the annular wall and an outer periphery connected to the corresponding flange which extends radially outwards from this outer periphery, characterized in that said first web has a generally frustoconical shape which flares from its inner periphery to its outer periphery on the side of said second web. In the current art, the webs each have a perfectly radial orientation or have a frustoconical shape inflaring on the side opposite the shaft. This particular shape causes the aforementioned phenomenon of axial contraction of the shaft under centrifugal effect. To counter this phenomenon and even reverse it, the frustoconicity of at least one of the shaft webs is reversed, this web now flaring on the side of the other web of the shaft, i.e. on the shaft side. In operation, the centrifugal forces will now cause an axial extension of this web and therefore of the shaft. This axial extension opposes the axial displacement imposed by the engine in operation. The reversed frustoconicity web can be configured, and in particular dimensioned, so that the axial extension exactly compensates for the axial displacement imposed by the engine. Thus, the shaft line of the turbomachine is not subjected to axial stress and is not likely to generate misalignments in the gear teeth. In the case where the axial extension generated by one of the webs atreversed frustoconicity of the shaft would not be sufficient to compensate for the axial displacement imposed by the motors, the two sails of the shaft could have a reversed frustoconicity so that the sum of their axial extensions compensates for this axial displacement. The coupling shaft according to the invention may comprise one or more of the following characteristics, taken in isolation from each other, or in combination with each other: - said second sail has a generally frustoconical shape which flares from its internal periphery to its external periphery on the side of said first sail; said second sail has a generally frustoconical shape which flares from its internal periphery to its external periphery on the side opposite said first sail; - the second sail forms an angle b with a plane perpendicular to the longitudinal axis and passing through the second flange, this angle b being between 1 and 40°, and preferably between 1 and 30°, and being measured in another planepassing through the longitudinal axis; - the first web forms an angle a with a plane perpendicular to the longitudinal axis and passing through the first flange, this angle a being between 1 and 40°, and preferably between 1 and 30°, and being measured in another plane passing through the longitudinal axis; - the angles a and b are different; - the angles a and b are identical; -- the angle a is positive or negative; -- the angle b is positive or negative; - each of said first and second webs has a minimum thickness less than a minimum thickness of said wall; - the annular wall comprises an inner annular surface which is connected by a first radius of curvature to a first surface of each of the webs, and an outer annular surface which is connected by a second radius of curvature to a second surface of each of the webs, the first and second radii of curvature at the first web being different and / or the first and second radii of curvature at the secondsail being different. The present invention also relates to an aircraft turbomachine, comprising three shafts extending along and around a longitudinal axis and integral in rotation around this axis, these three shafts comprising: - an upstream shaft comprising an annular wall, a downstream longitudinal end of which is connected by an annular sail to an annular flange, this sail comprising an internal periphery connected to the annular wall and an external periphery connected to the flange which extends radially outwards from this external periphery, - a downstream shaft comprising an annular wall, an upstream longitudinal end of which is connected by an annular sail to an annular flange, this sail comprising an internal periphery connected to the annular wall and an external periphery connected to the flange which extends radially outwards from this external periphery, and - a coupling shaft as described above interposed between the upstream anddownstream, the flanges of the coupling shaft being respectively fixed to the flanges of the upstream and downstream shafts. The turbomachine according to the invention may comprise one or more of the following characteristics, taken in isolation from one another, or in combination with one another: - the first flange is fixed to the flange of the upstream shaft, the web of this upstream shaft having a generally frustoconical shape which flares from its internal periphery to its external periphery on the side opposite the coupling shaft, or - the first flange is fixed to the flange of the downstream shaft, the web of this downstream shaft having a generally frustoconical shape which flares from its internal periphery to its external periphery on the side opposite the coupling shaft; - the internal peripheries of the first web and of the web facing the upstream or downstream shaft are separated from one another by a distance D1, and the external peripheries of the first web and of the web facing the upstream or downstream shaft areseparated from each other by a distance D2, D1 being less than D2; - the second flange is fixed to the flange of the downstream shaft, the web of this upstream shaft having a generally frustoconical shape which flares from its internal periphery to its external periphery on the side opposite the coupling shaft, or - the second flange is fixed to the flange of the upstream shaft, the web of this upstream shaft having a generally frustoconical shape which flares from its internal periphery to its external periphery on the side opposite the coupling shaft; - the second flange is fixed to the flange of the downstream shaft, the web of this downstream shaft having a generally truncated cone shape which flares from its internal periphery to its external periphery on the side of the coupling shaft, or - the second flange is fixed to the flange of the upstream shaft, the web of this upstream shaft having a generally truncated cone shape which flares from its internal periphery to its external periphery on the side of the coupling shaft; - the downstream shaft is a turbine shaft or coupled to a turbine shaft, and the downstream shaft is an input shaft of a mechanical reducer or coupled to such an input shaft, the reducer comprising a flexible output coupled to a propeller shaft; -- the or each sail (with reversed frustoconical shape) comprises two annular surfaces, respectively upstream and downstream, which each have a general frustoconical shape flared on the side of the other sail; in other words, the or each sail and their annular surfaces have the same reversed frustoconical shape; -- the annular surfaces of the or each sail (with reversed frustoconical shape) extend over at least 50%, and preferably at least 70%, of a height or radial dimension of this sail. Brief description of the figures Other characteristics and advantages will emerge from the following description of a non-limiting embodiment of the invention with reference to the appended drawings in which: [Fig. 1] Figure 1 is a schematic axial sectional viewof an aircraft turbomachine, [Fig.2] Figure 2 is a larger-scale schematic view of a part of Figure 1, [Fig.3] Figure 3 is a view similar to that of Figure 2, [Fig. 4] Figure 4 is a schematic half-view in axial section of a coupling device of the flex coupling type, [Fig. 5] Figure 5 is a schematic axial section view of a line of shafts and a coupling shaft according to the prior art, [Fig. 6] Figure 6 is a schematic axial section view of a line of shafts and a coupling shaft according to an embodiment of the invention, [Fig.7] Figure 7 is a larger-scale schematic view of a part of Figure 6, and [Fig. 8] Figure 8 is a schematic axial sectional view of a line of shafts and a coupling shaft according to an alternative embodiment of the invention. Detailed description of the invention Referring to Figure 1, we see an aircraft turbomachine 1, which comprises, in a mannerconventional, a fan S, a low-pressure compressor 1a, a high-pressure compressor 1b, a combustion chamber 1c, a high-pressure turbine 1d and a low-pressure turbine 1e. The rotors of the high-pressure compressor 1b and the high-pressure turbine 1d are connected by a high-pressure shaft 5 and together form a high-pressure (HP) body. The rotors of the low-pressure compressor 1a and the low-pressure turbine 1e are connected by a low-pressure shaft 4 and together form a low-pressure (LP) body. The fan S is, for its part, carried by a fan shaft 3 which is connected by a reducer 7 to the LP shaft 4. The HP and LP shafts 4, 5 extend along an axis A which is the axis of rotation of the turbomachine 1. In the remainder of the description, the notions of longitudinal or radial, and internal or external, are relative to this axis A. The turbomachine 1 comprises structural casings. The HP body is held by two structural casings: the casinginter-compressor and the inter-turbine casing, and the LP body is held by at least two structural casings: the intermediate casing 2 and the inter-turbine casing and / or the exhaust casing 6. The intermediate casing 2 supports bearings of the LP turbine shaft 4 which are housed in a front or upstream enclosure denoted E1. The exhaust casing 6 supports bearings of the LP turbine shaft 4 which are housed in a rear or downstream enclosure denoted E2. The enclosures E1, E2 are generally delimited by bearing supports. The reducer 7 is here of the epicyclic type. Figure 2 shows very schematically the size of the reducer 7. The reducer 7 comprises an input shaft 8 extending upstream of the LP shaft 4 and which is guided by a downstream bearing 10. The input shaft 8 comprises an axial end, here upstream, engaged in the reducer 7 and meshed with a sun gear of the reducer, which is itself meshed with the satellites of the reducer. The input shaft 8 has itsdownstream end which is meshed with a shaft 15 of the low pressure compressor which is itself meshed with the shaft 4. The bearing 10 extends here around the shaft 15. The torque at the output of the reducer 7 is transmitted to the fan shaft 3, by a conventional connection, known to those skilled in the art, such as for example a fixing of this fan shaft on the planet carrier forming an output shaft 9 of the reducer, in the case of an epicyclic reducer. In the case of a planetary reducer, the fan shaft would be driven by the crown of the reducer 7, which is itself meshed with the satellites of the reducer. The reducer 7 is placed inside the front enclosure E1 for lubrication. The enclosure E1 comprises fixed walls and movable walls. The fixed walls of the enclosure E1 comprise an internal wall of the primary flow vein, an upstream annular support 11 of bearings 13, 14 and a downstream annular support 12 of bearing 10. The supports 11 and 12 extendtowards the inside of the turbomachine and respectively carry the bearings 13, 14 and the bearing 10. They provide the structure between the casings and the fixed external rings of the bearings 13, 10. The moving walls of the enclosure E1 comprise the input 8 and output 9 shafts. The bearings 10, 13, 14 are housed in the enclosure E1. Seals, not visible in the diagrams, are provided between the fixed and moving walls and are, for example, labyrinth seals, brush seals, segmented radial seals, etc. The bearings 10, 13 and 14 as well as the reducer 7 are lubricated for their proper operation. The oil is supplied by suitable means such as nozzles, oil supply ducts, etc. The bearing support 11 comprises ventilation holes which allow ventilation air to pass through the enclosure. The E1 enclosure is configured so that the air-oil mixture, which forms an oil mist inside the enclosure, is contained within the enclosure. Between the wallsrotor and stator of the enclosure, for example here at the upstream and downstream ends of the enclosure, seals (such as labyrinths) are placed to contain the oil, and an air circuit pressurizes these seals to prevent oil leaks. The enclosure E1 is then pressurized (air enters it continuously, pushing back the oil which could have escaped from the seals by capillarity) and the bearings operate in a medium of mixed oil and air. The bearings are supplied by a supply tube and the recovery is ensured by a specific recovery tube which extends along the X axis and inside the shaft line, in particular low pressure, as schematically illustrated by arrows in Figure 1. To avoid overpressure of the enclosure, and to allow a constant flow of incoming air, the interior of the enclosure is vented to a lower pressure than the pressure of the air entering the seals. This charged airof oil particles, which is discharged at a pressure well, must first be treated to recover almost all of the oil it carries. For this, the oiled air will be brought to an oil separator which will separate the air from the oil it carries and will reject the de-oiled air outside the engine. This is the principle of de-oiling an enclosure. In the present application, the term "shaft line" means a series of shafts which extend along the same axis and which are integral in rotation with each other. This line comprises at least two shafts and these shafts are connected together by a coupling device. In the context of the present invention, the couplings between the shafts of the shaft line are of the flex coupling type. A coupling device 20 of the flex coupling type is illustrated in Figures 1 and 2 and makes it possible to connect two shafts 8a, 8b. In the configuration of Figure 3, the shaft line comprises three shafts 8a, 8b, 8c which are connected to each otherothers by two coupling devices 20. The shaft 8a is an upstream shaft, the shaft 8c is a downstream shaft, and the shaft 8b is an intermediate shaft called a coupling shaft. Each coupling device 20 is used to ensure the transmission of torque between two shafts of the low pressure shaft line at the input of the reducer 7. Figures 4 and 5 represent a more concrete example of a coupling device 20. The upstream shaft 8a comprises an annular wall 8aa which comprises at its upstream end a toothing 21 for meshing with the sun gear of the reducer 7. The downstream end of the wall 8aa is connected to an annular web 22 which extends radially outwards and is connected to an annular flange 24 which itself extends radially outwards. The flange 24 has an axial thickness greater than that of the rest of the web 22 which has a certain flexibility, particularly in bending. The downstream shaft 8b comprises an annular wall 8ba whose endupstream is connected to a web 26 which extends radially outwards and is connected to an annular flange 28 which itself extends radially outwards. The flange 28 also has an axial thickness greater than that of the web 26 which has a certain flexibility, particularly in bending. The webs 22, 26 each have a radial orientation with respect to the axis A and are substantially parallel and axially spaced from each other by a predetermined distance sufficient to allow the shafts 8a, 8b to work in bending. The flanges 24, 28 comprise aligned axial orifices 30 for the passage of screw-nut type fixing means. The screws 32 here have heads bearing axially on a downstream radial face of the flange 28 and receive nuts bearing axially on an upstream radial face of the flange 24. The assembly formed by the webs 22, 26, the flanges 24, 28 and the screws 32 form a connection of the “flex-coupling” type which has a certain flexibility and allowsrelative displacements between the shafts 8a, 8b. Figure 5 shows a line of shafts with three shafts 8a, 8b, 8c, as in Figure 3. As mentioned above, the shafts 8a, 8b, 8c are respectively an upstream shaft, an intermediate or coupling shaft and a downstream shaft. This Figure 3 illustrates the technique prior to the present invention in which the webs of the shafts have perfectly radial orientations, that is to say that these webs extend in planes perpendicular to the axis A, or have conventional frustoconicalities, that is to say a frustoconical shape which flares out on the side opposite the shaft. Thus, the upstream shaft 8a has a web 22 at its downstream end which flares out on the side of the coupling shaft 8b, that is to say that the internal periphery of the web 22 is located upstream of its external periphery. The coupling shaft 8b has a web 26 at its upstream end which flares out towards the input shaft 8a, i.e. the inner periphery of the web 26 islocated downstream of its outer periphery. The coupling shaft 8b has a web 34 at its downstream end which flares out towards the output shaft 8c, i.e. the inner periphery of the web 34 is located upstream of its outer periphery. And the output shaft 8c has a web 36 at its upstream end which flares out towards the coupling shaft 8b, i.e. the inner periphery of the web 36 is located downstream of its outer periphery. Figures 6 and 7 illustrate a first embodiment of the invention and Figure 8 illustrates an alternative embodiment of the invention. In Figures 6 and 7, one of the webs 26, 34 of the coupling shaft 8b has a reversed frustoconicity and, in Figure 8, both webs 26, 34 of the coupling shaft 8b have a reversed frustoconicity. The coupling shaft 8b extends along and around the longitudinal axis A and has an annular or tubular wall 8ba which includes a first longitudinal end connected by a firstannular web 34, here downstream, to a first annular flange 38, and a second longitudinal end 26, here upstream, connected by a second annular web 26 to a second annular flange 28. Each of the first and second webs 34, 26 has an internal periphery connected to the annular wall 8b1 and an external periphery connected to the corresponding flange 38, 28 which extends radially outwards from this external periphery. The first web 34 has a generally truncated cone shape which flares from its inner periphery to its outer periphery on the side of the second web 26. The first web 34 therefore has a reversed truncated cone shape compared to that of Figure 5. The second web 26 has a generally truncated cone shape which flares from its inner periphery to its outer periphery on the side opposite the first web 34. The second web 26 therefore has a conventional truncated cone shape similar to that of Figure 5. As can be seen in Figures 6 and 7, the first web 34 forms aangle a with a plane P1 perpendicular to the axis A and passing through the first flange 38. This angle a is between 1 and 40°, and preferably between 1 and 30°, and is measured in another plane P2 passing through the axis A. The second web 26 forms an angle b with a plane P3 perpendicular to the axis A and passing through the second flange 28. This angle b is between 1 and 40°, and preferably between 1 and 30°, and is measured in the plane P2 passing through the axis A. In the example shown, the angles a and b are different and in particular reversed due to the inversion of frustoconicity. It is also noted in the example shown that each of the webs 26, 34 has a minimum thickness E1 less than a minimum thickness E2 of the wall 8ba. The annular wall 8ba comprises an inner annular surface 8ba1 which is connected by a first radius of curvature R1 to a first surface 26a, 34a of each of the webs 26, 34, and an outer annular surface 8ba2 which is connected by a second radius ofcurvature R2 to a second surface 26b, 34b of each of the webs 26, 34. The radii of curvature R1, R2 at the first web 34 may be different. The radii of curvature R1, R2 at the second web 26 may be different. The upstream shaft 8a comprises an annular wall 8aa, a downstream longitudinal end of which is connected by an annular web 22 to an annular flange 24. This web 22 comprises an inner periphery connected to the annular wall 8aa and an outer periphery connected to the flange 24 which extends radially outwards from this outer periphery. The web 22 of this upstream shaft 8a has a generally frustoconical shape which flares from its inner periphery to its outer periphery on the side of the coupling shaft 8b. This web 22 therefore has a conventional frustoconical shape. The web 22 forms an angle b' with the plane P3, which may be equal to or different from the angle b. The web 22 may have a minimum thickness less than a minimum thickness of the wall8aa. The upstream shaft 8a may comprise at its upstream end a toothing 21 (herringbone or helical) for meshing with the sun gear of the reducer 7. The annular wall 8aa comprises an internal annular surface 8aa1 which is connected by a first radius of curvature R3 to a first surface 24a of the web 22, and an external annular surface 8aa2 which is connected by a second radius of curvature R4 to a second surface 24b of the web 22. The radii of curvature R3, R4 may be different. The flanges 24 and 28 are applied axially against each other and tightened together by means of the screw-nut type or the like, as mentioned above. The inner peripheries of the sails 26, 24, which are axially opposite each other, are separated from each other by a distance D3, and the outer peripheries opposite these sails are separated from each other by a distance D4. Due to their frustoconicalities, D3 is here greater than D4. The downstream shaft 8c has an annular wall8ca, one downstream longitudinal end of which is connected by an annular web 36 to an annular flange 40. This web 36 has an internal periphery connected to the annular wall 8ca and an external periphery connected to the flange 40 which extends radially outwards from this external periphery. The web 36 of this downstream shaft 8c has a generally frustoconical shape which flares from its internal periphery to its external periphery on the side opposite the coupling shaft 8b. This web 22 therefore has an inverted frustoconical shape, as is the case with its facing web 34. The flanges 38, 40 are applied axially against each other and tightened together by screw-nut type means or the like, as mentioned above. The inner peripheries of the sails 34, 36, which are axially opposite each other, are separated from each other by a distance D1, and the outer peripheries opposite these sails are separated from each other by a distance D2. Due to theirfrustoconicities, D1 is here less than D2. The web 36 forms an angle a' with the plane P1, which may be equal to or different from the angle a. The web 36 may have a minimum thickness less than a minimum thickness of the wall 8ca. The downstream shaft 8c may comprise at its downstream end rectilinear grooves 21' for meshing with another shaft for example. The annular wall 8ca comprises an internal annular surface 8ca1 which is connected by a first radius of curvature R5 to a first surface 36a of the web 36, and an external annular surface 8aa2 which is connected by a second radius of curvature R6 to a second surface 36b of the web 36. The radii of curvature R5, R6 may be different. In the drawings, it can be seen that the surfaces 34a, 34b, 36a, 36b of each of the webs 34, 36 with reversed frustoconical shape each have a general frustoconical shape that flares out on the side of the other web 34, 36. In other words, each web 34, 36 and their annular surfaces have the same frustoconical shape.inverted. The surfaces 34a, 34b, 36a, 36b extend over at least 50%, and preferably at least 70%, of a height or radial dimension of the corresponding web 34, 36. In the variant embodiment of Figure 8, the coupling device 20 between the coupling shaft 8b and the downstream shaft 8c is identical to that of Figures 6 and 7. The second web 26 has a generally frustoconical shape which flares from its internal periphery to its external periphery on the side of the web 34. The second web 26 therefore has an inverted frustoconicality. The second web 26 forms an angle b with a plane P3 perpendicular to the axis A and passing through the second flange 28. This angle b is between 1 and 40°, and preferably between 1 and 30°, and is measured in the plane P2 passing through the axis A. In the example shown, the angles a and b may be different or identical. The web 26 may have a minimum thickness E1 less than a minimum thickness E2 of the wall 8ba. The radii of curvature R1, R2 of theweb 26 may be different. The web 22 of the upstream shaft 8a has a generally truncated cone shape which flares from its internal periphery to its external periphery on the side opposite the coupling shaft 8b. This web 22 therefore has an inverted truncated cone. The web 22 forms an angle b with the plane P3, which may be equal to or different from the angle b. The web 22 may have a minimum thickness less than a minimum thickness of the wall 8aa. The upstream shaft 8a may comprise at its upstream end a toothing 21 (herringbone or helical) for meshing with the sun gear of the reducer 7. The radii of curvature R3, R4 may be different. The flanges 24 and 28 are applied axially against each other and tightened together by means of the screw-nut type or the like, as mentioned above. The internal peripheries of the sails 26, 24, which are axially opposite each other, are separated from each other by a distance D3, and the external peripheries oppositethese sails are separated from each other by a distance D4. Due to their frustoconicalities, D3 is here less than D4. In the examples illustrated in the drawings, each coupling device has a support plane of its flanges which is perpendicular to the axis A and which passes substantially between the sails connected to these flanges, whether these sails have a conventional or inverted frustoconicality. Although it is preferable for two sails with a conventional frustoconicality to be associated by their flanges, and for two sails with an inverted frustoconicality to be associated by their flanges, it is not to be excluded to associate a sail with a conventional frustoconicality with a sail with an inverted frustoconicality. In yet another variant not shown, the coupling device 20 with inverted frustoconicalities of figures 6 and 7 could be upstream of the coupling shaft 8b rather than downstream. In operation, the inverted frustoconicalities of the or each coupling device 20 illustrated in Figures 6 to 8cause an axial extension or expansion of the coupling shaft 8b and the shaft line. This axial extension is able to compensate for the axial displacement imposed by the motor in operation so as to limit or even eliminate the risk of axial misalignment of the gear teeth. Naturally, the coupling shaft 8b according to the invention could be used in another context in which it would not necessarily be coupled to a gear reducer for example.

Claims

CLAIMS 1. Coupling shaft (8b) for an aircraft turbomachine (1), this shaft (8b) extending along and around a longitudinal axis (A) and comprising an annular wall (8ba) which comprises a first longitudinal end connected by a first annular web (34) to a first annular flange (38), and a second longitudinal end connected by a second annular web (26) to a second annular flange (28), each of the first and second webs (26, 34) comprising an inner periphery connected to the annular wall (8ba) and an outer periphery connected to the corresponding flange (28, 38) which extends radially outwards from this outer periphery, characterized in that said first web (34) has a generally frustoconical shape which flares from its inner periphery to its outer periphery on the side of said second web (26). 2.Coupling shaft (8b) according to claim 1, wherein said second web (26) has a generally frustoconical shape which flares from its inner periphery to its outer periphery on the side of said first web (34).

3. Coupling shaft (8b) according to claim 1, wherein said second web (26) has a generally frustoconical shape which flares from its inner periphery to its outer periphery on the side opposite said first web (34).

4. Coupling shaft (8b) according to claim 2 or 3, wherein the second web (26) forms an angle b with a plane (P3) perpendicular to the longitudinal axis (A) and passing through the second flange (28), this angle b being between 1 and 40°, and preferably between 1 and 30°, and being measured in another plane (P2) passing through the longitudinal axis (A). 5.Coupling shaft (8b) according to one of the preceding claims, in which the first web (34) forms an angle α with a plane perpendicular to the longitudinal axis (A) and passing through the first flange (38). this angle a being between 1 and 40°, and preferably between 1 and 30°, and being measured in another plane (P2) passing through the longitudinal axis (A).

6. Coupling shaft (8b) according to claim 5 when it depends on claim 4, in which the angles a and b are different.

7. Coupling shaft (8b) according to claim 5 when it depends on claim 4, in which the angles a and b are identical.

8. Coupling shaft (8b) according to one of the preceding claims, in which each of said first and second webs (26, 34) has a minimum thickness (E1) less than a minimum thickness (E2) of said wall (26, 34). 9.Coupling shaft (8b) according to one of the preceding claims, wherein the annular wall (8ba) comprises an inner annular surface (8ba) which is connected by a first radius of curvature (R1) to a first surface (26a, 34a) of each of the webs (26, 34), and an outer annular surface (8ba2) which is connected by a second radius of curvature (R2) to a second surface (26b, 34b) of each of the webs (26, 34), the first and second radii of curvature (R1, R2) at the first web (34) being different and / or the first and second radii of curvature (R1, R2) at the second web (26) being different.Aircraft turbomachine (1), comprising three shafts (8a, 8b, 8c) extending along and around a longitudinal axis (A) and integral in rotation around this axis, these three shafts (8a, 8b, 8c) comprising: - an upstream shaft (8a) comprising an annular wall (8aa) of which a downstream longitudinal end is connected by an annular web (22) to an annular flange (24), this web (22) comprising an internal periphery connected to the annular wall and an external periphery connected to the flange (24) which extends radially outwards from this external periphery, - a downstream shaft (8c) comprising an annular wall of which an upstream longitudinal end is connected by an annular web (36) to an annular flange (40), this web (36) comprising an internal periphery connected to the. annular wall and an outer periphery connected to the flange (40) which extends radially outwards from this outer periphery, and - a coupling shaft (8b) according to one of the preceding claims interposed between the upstream and downstream shafts (8a, 8c), the flanges (28, 38) of the coupling shaft (8b) being respectively fixed to the flanges (24, 40) of the upstream and downstream shafts (8a, 8c).

11. Turbomachine (1) according to claim 10, in which: - the first flange (38) is fixed to the flange (24) of the upstream shaft (8a), the web (22) of this upstream shaft (8a) having a generally frustoconical shape which flares from its internal periphery to its external periphery on the side opposite the coupling shaft (8b), or - the first flange (38) is fixed to the flange (40) of the downstream shaft (8c), the web (36) of this downstream shaft (8c) having a generally frustoconical shape which flares from its internal periphery to its external periphery on the side opposite the coupling shaft (8b). 12.Turbomachine (1) according to claim 11, in which the internal peripheries of the first web (34) and of the web (22, 36) facing the upstream or downstream shaft (8a, 8c) are separated from each other by a distance D1, and the external peripheries of the first web (34) and of the web (22, 26) facing the upstream or downstream shaft (8a, 8c) are separated from each other by a distance D2, D1 being less than D2. 13.Turbomachine (1) according to one of claims 10 to 12, the coupling shaft (8b) being as defined in claim 2, in which: - the second flange (28) is fixed to the flange (40) of the downstream shaft (8c), the web (36) of this downstream shaft (8c) having a generally frustoconical shape which flares from its internal periphery to its external periphery on the side opposite the coupling shaft (8b), or - the second flange (28) is fixed to the flange (24) of the upstream shaft (8a), the web (22) of this upstream shaft (8a) having a generally frustoconical shape which flares from its internal periphery to its external periphery on the side opposite the coupling shaft (8b).

14. Turbomachine (1) according to one of claims 10 to 12, the coupling shaft (8b) being as defined in claim 3, in which: - the second flange (28) is fixed to the flange (40) of the downstream shaft (8c), the web (36) of this downstream shaft (8c) having a generally frustoconical shape which flares from its internal periphery to its external periphery on the side of the coupling shaft (8b), or - the second flange (28) is fixed to the flange (40) of the upstream shaft (8a), the web (22) of this upstream shaft (8a) having a generally frustoconical shape which flares from its internal periphery to its external periphery on the side of the coupling shaft (8b). 15.Turbomachine (1) according to one of claims 10 to 14, in which the downstream shaft (8c) is a turbine shaft or coupled to a turbine shaft, and the upstream shaft (8a) is an input shaft of a mechanical reducer (7) or coupled to such an input shaft, the reducer (7) comprising a flexible output coupled to a propeller shaft.

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