Method for mounting a rotor between two bearings

Abstract

The invention relates to a method of mounting (100) an assembly (60) of a propulsion system (2) for an aircraft, said assembly (60) comprising a rotor (62) rotating about a longitudinal axis (X) relative to a fixed structure (64), said rotor (62) comprising an upstream section (56) and a downstream section (54) connected to each other by a transmission system (50), the upstream and downstream sections inducing an axial force on the transmission system during the operation of the propulsion system (2), the upstream section (56) being connected to the fixed structure (64) by an upstream bearing (70) and the downstream section (54) being connected to the fixed structure (64) by a downstream bearing (80), the upstream bearing (70) comprising an upstream inner ring (72) and an upstream outer ring (74), the downstream bearing (80) comprising a downstream inner ring (82) and a downstream outer ring (84),said method (100) comprising the following steps: - S1: determine a maximum value of the axial force that must not be exceeded when the propulsion system (2) is in operation; - S2: determine at least one target parameter; - S3: configure at least one of the upstream bearing (70) and the downstream bearing (80) so as to obtain said axial positioning at rest of the inner ring (72, 82) of the bearing (70, 80) relative to its outer ring (74, 84), and / or dimension the length of the rotor (62) between the two bearings (70, 80) so that the rotor (62) is subjected to the axial stress at rest once mounted between the two bearings while the propulsion system (2) is at rest. Figure for the abstract: Fig. 4,

Description

Title of the invention: Method for mounting a rotor between two bearings Scope of the invention

[0001] The present invention relates to a method of assembling a propulsion system assembly, a propulsion system comprising such an assembly, and also an aircraft comprising the propulsion system. Technological background

[0002] Various types of aircraft propulsion systems are known, including turbofan engines, which consist of a primary unit of the gas generator type and a fan driven by a turbine shaft of the primary unit. Newer generations of these turbofan engines, particularly those with a high bypass ratio, generally include a mechanical gearbox to drive the fan shaft.

[0003] Typically, the purpose of the gearbox is to transform the relatively high rotational speed of the turbine shaft into a slower rotational speed for the blower shaft. The turbine shaft (or downstream shaft) and the blower shaft (or upstream shaft), connected via the gearbox, form a rotor of the gas generator.

[0004] Most often, the gearboxes used in such turbomachines are epicyclic gear gearboxes comprising a central pinion, called the sun gear, a ring gear coaxial with the sun gear, surrounding the latter, and pinions, called planet gears, which mesh between the sun gear and the ring gear and are held by a frame called a planet carrier. Conventionally, the sun gear is driven in rotation by the turbine shaft and the fan shaft is driven in rotation by the planet carrier (when the gearbox is in an epicyclic configuration) or by the ring gear (when the gearbox is in a planetary configuration).

[0005] During the operation of the turbomachine, axial movements of the rotor are unavoidable, which in turn cause axial movements of the turbine shaft and the fan shaft relative to the gearbox. In order to limit the axial movements of the rotor relative to the gearbox, the latter can be made with herringbone teeth.

[0006] Furthermore, it is also known to position, around the rotor, a bearing upstream of the gearbox and a bearing downstream of the gearbox. These bearings generally comprise an inner ring surrounding the rotor and an outer ring fixed to the stator. During the operation of the turbomachine, the rotor, which is in a "hot" part of the turbomachine, undergoes contractions and expansions, particularly along the axial direction, due to temperature changes, while the stator, which is in a part "Cold" means it undergoes less expansion and contraction. The stator thus prevents axial movement of the rotor, which leads to induced stresses at the level of the reduction gear, particularly at the level of the solar element, and more specifically at the level of the gear teeth.

[0007] One way to limit these forces is to position one or more flexible coupling devices on the rotor, typically on the rotor turbine shaft. However, these devices can be large and / or bulky and also make the rotor manufacturing more complex.

[0008] Another way to limit these stresses on the gearbox is to allow some clearance in the bearings, permitting a slight movement of the rotor relative to the stator. For example, in the case of ball bearings with circular raceways, it is possible to create a radius of curvature for each ring that is slightly larger than the radius of the balls. However, this clearance is generally too small to absorb the expansion / contraction differences between the rotor and the stator. Description of the invention

[0009] One objective of the invention is to limit the internal stresses on a rotor mounted between two thrust bearings, that is, two bearings capable of absorbing axial forces from the rotor, when the turbomachine is in operation. For example, one objective of the invention is to reduce the stresses experienced by a gearbox mounted between two shafts whose axial movements are limited by thrust bearings.

[0010] To this end, the invention has as its first object a method for assembling an aircraft propulsion system, said assembly comprising a rotor rotating about a longitudinal axis (X) relative to a fixed structure, said rotor comprising an upstream section and a downstream section connected to each other by a transmission system, the upstream and downstream sections inducing an axial force on the transmission system during the operation of the propulsion system, the upstream section being connected to the fixed structure by an upstream bearing and the downstream section being connected to the fixed structure by a downstream bearing, the upstream bearing comprising an upstream inner ring and an upstream outer ring, the downstream bearing comprising a downstream inner ring and a downstream outer ring, said method comprising the following steps: - SI: determine a maximum value of the axial force which must not be exceeded when the propulsion system is in operation; - S2: Determine at least one target parameter from among: a) an axial positioning at rest of the inner ring of one of the bearings relative to its outer ring, and b) a rest axial stress value to which the rotor must be subjected between the upstream and downstream bearings when the propulsion system is at rest, so that, during the operation of the propulsion system, the axial force does not exceed the determined maximum value; - S3: configure at least one of the upstream and downstream bearings so as to obtain said axial positioning at rest of the inner ring of the bearing relative to its outer ring, and / or dimension the length of the rotor between the two bearings so that the rotor is subjected to axial stress at rest once mounted between the two bearings while the propulsion system is at rest.

[0011] The assembly method according to the first object makes it possible to prevent the axial force induced on the transmission system by the upstream and downstream sections of the rotor, during the operation of the propulsion system, from exceeding the maximum determined value.

[0012] In this description, the expression "at rest" means that the propulsion system is not in operation. For example, the propulsion system may be being assembled, or operational but stopped.

[0013] The method makes it possible to determine a maximum axial force that can be exerted on the transmission system by the upstream and downstream sections of the rotor, taking into account various parameters of the assembly such as the type of bearings, the dimensions of the bearings and the rotor, and / or the type and dimensions of the transmission system. These parameters may also include the flexibility values ​​of the rotor and stator, the power passing through the gearbox, in particular the power generated by the turbine and transmitted to the blower, and / or the axial forces passing through the rotor during the operation of the propulsion system. The axial forces passing through the gearbox correspond in particular to the rotor displacements under thermal expansion prevented by the presence of the two bearings, and to the tensile or compressive forces generated by the blower, a compressor, and / or a turbine.

[0014] The maximum force value can also be determined by taking into account the conditions to which the assembly is subjected when the propulsion system is in operation, such as temperature, the surrounding fluid(s), or pressure. The axial force in operation corresponds in particular to a restrained displacement due to the presence of the upstream and downstream bearings. The relationship between the restrained displacement of the rotor and the resulting force is determined by the total axial flexibility of the rotor. The mounting method makes it possible to define the cold position of the rotor and / or to introduce a restrained axial displacement of the rotor at rest in order to obtain the desired value of the restrained axial displacement of the rotor in operation and thus the resulting force.

[0015] Once the maximum axial force value in operation has been determined, the determination of at least one of the target parameters makes it possible to achieve an optimized assembly so that the maximum force value is not exceeded during operation.

[0016] The transmission system may in particular be a reducer which may be planetary or epicyclic.

[0017] Regarding the upstream bearing, the upstream inner ring is integral with the rotor and is thus rotatably mounted around the axis (X). The position of the upstream inner ring around the upstream section can be maintained by means of one or more raised features on the upstream section forming one or more stops for the upstream inner ring. The upstream outer ring is connected to the fixed structure forming the stator.

[0018] Regarding the downstream bearing, the downstream inner ring is integral with the rotor and is thus rotatably mounted around the axis (X). The position of the downstream inner ring around the downstream section can be maintained by means of one or more raised features on the downstream section forming one or more stops for the downstream inner ring. The downstream outer ring is connected to the fixed structure.

[0019] The upstream bearing and the downstream bearing may be identical or different and are preferably thrust bearings, that is to say bearings that can constitute axial stops to prevent axial displacement of the rotor they guide, such as ball bearings or tapered roller bearings.

[0020] According to embodiments that can be taken alone or in combination: - the configuration of at least one of the upstream and downstream bearings includes determining an axial position of at least one of the downstream inner ring relative to the downstream outer ring and of the upstream inner ring relative to the upstream outer ring; the axial position being able to be an axial alignment of the inner ring relative to the outer ring, an upstream axial offset of the inner ring relative to the outer ring, or a downstream axial offset of the inner ring relative to the outer ring; determining the position of the upstream inner ring relative to the upstream outer ring and / or of the downstream inner ring relative to the downstream outer ring allows choosing the value of the clearance at the upstream and / or downstream bearing, the clearance being the possibility of axial displacement of the inner ring relative to the outer ring, and the amplitude of this displacement; - the positioning of the inner ring of one of the bearings in an axially offset manner relative to the corresponding outer ring, and positioning the other inner ring in an axially aligned manner relative to the corresponding outer ring; thus, one of the bearings has maximum axial play and allows the rotor a slight upstream and downstream movement while the other bearing allows less, or even zero, movement in one of the upstream and downstream directions depending on whether the axial offset is upstream or downstream, respectively; - Positioning the inner ring of one of the bearings axially offset from the corresponding outer ring involves placing a shim between the inner ring and a raised area on the rotor; this allows for the selection of the value of the axial offset by choosing the size of the shim in order to obtain the desired axial clearance between the inner ring and the outer ring; - dimensioning the length of the rotor between the two bearings involves applying a thermal differential between the rotor and the stator at the time of positioning the rotor between the two bearings, then stabilizing the assembly at the same temperature in a rest position; the rotor can thus be compressed or expanded relative to the stator, which results in a temporary change in the length of the rotor, by expansion or contraction; the rotor can then be placed between the bearings and when its temperature stabilizes at the same temperature as the stator, the rotor is under stress; the term "under stress" means that, in the rest position, the rotor is stretched or compressed relative to an initial state in which the rotor has not undergone any stress, the rest position being the position of the rotor once mounted between the two bearings and when the propulsion system is not in operation; - the application of a thermal differential between the rotor and the stator involves applying to the rotor a temperature higher than the stabilization temperature, so that the rotor is expanded, then positioning the rotor between the two bearings when it is in this expanded state; - Applying a thermal differential between the rotor and the stator involves applying a temperature to the rotor that is lower than its stabilization temperature, so that the rotor contracts, and then positioning the rotor between the two bearings when it is in this contracted state; and - the upstream bearing and the downstream bearing are bearings capable of taking axial forces such as thrust bearings.

[0021] The invention has as its second object an aircraft propulsion system assembly, said assembly comprising a rotor rotating about a longitudinal axis (X) relative to a fixed structure, said rotor comprising an upstream section and a downstream section connected to each other by a transmission system, the upstream section being connected to the fixed structure by an upstream bearing and the downstream section being connected to the fixed structure by a downstream bearing, the upstream bearing comprising an upstream inner ring and an upstream outer ring, the downstream bearing comprising a downstream inner ring and a downstream outer ring, in which the inner ring of one of the bearings is axially offset relative to the corresponding outer ring, the inner ring of the other bearing being axially aligned relative to the corresponding outer ring.

[0022] The assembly makes it possible to limit the axial force induced by the rotor on the transmission system during the operation of the propulsion system, and in particular to allow this axial force not to exceed a predetermined maximum value.

[0023] Furthermore, the rotor of the assembly can be under axial stress such that its rest length between the upstream and downstream bearings, when the rotor is positioned between these two bearings once the assembly is complete and the propulsion system is stopped, are different from an initial length when it is not under stress.

[0024] The rest length is the length when the rotor is positioned between the two bearings once the assembly is complete and is in a state of stress, in particular when the rotor exerts a positive or negative force on the bearings.

[0025] The invention has as its third object a propulsion system for an aircraft comprising an assembly according to the second object or an assembly obtained according to the process of the first object.

[0026] The invention has as its fourth object an aircraft comprising a propulsion system according to the third object. Brief description of the Figures

[0027] Other features and advantages of the invention will become apparent from the following description, given solely by way of example and with reference to the accompanying drawings, in which:

[0028] Fig. 1 schematically represents a complete top view of an aircraft according to one embodiment of the invention; Figure [Fig. 2] schematically represents a partial and cross-sectional view of a propulsion system according to one embodiment of the invention; Fig. 3 schematically represents a partial and perspective view of a reducer of the propulsion system of Fig. 2; Fig. 4 schematically represents a partial and cross-sectional view of an assembly of the propulsion system of Fig. 2 according to one embodiment; Figure 5 is a flowchart representing a method for assembling the entire set of the [Fig.4] according to one embodiment of the invention; Fig. 6a, Fig. 6b, and Fig. 6c schematically represent a partial and cross-sectional view of bearings configured according to different embodiments of the invention; Figure 7 schematically represents a partial and cross-sectional view of a bearing mounted according to one embodiment of the invention; Figures [Fig. 8a], [Fig. 8b], and [Fig. 8c] schematically represent a partial, cross-sectional view of propulsion system assemblies configured according to different embodiments of the invention; and Figures [Fig.9a], [Fig.9b], and [Fig.9c] schematically represent a partial and cross-sectional view of propulsion system assemblies configured according to different embodiments of the invention. Detailed description of an example of implementation

[0029] The aircraft 1 shown in [Fig. 1] comprises propulsion systems 2 for the propel.

[0030] In the example shown, aircraft 1 is an airplane. It conventionally comprises a fuselage 4, a tail assembly 6, and two wings 18. The propulsion systems 2 are two in number and are each housed under a respective wing 18. Alternatively (not shown), the propulsion systems are arranged along the fuselage 4, for example, near the tail assembly 6. In yet another alternative (also not shown), aircraft 1 comprises a single propulsion system or at least three propulsion systems.

[0031] One of the propulsion systems 2 is shown in [Fig.2].

[0032] As can be seen in this figure, the propulsion system 2 is a double turbomachine flow comprising a blower 12 having a plurality of blades 14 extending radially from a hub (not shown). However, the invention can also be applied to a triple-flow turbomachine.

[0033] During the operation of the turbomachine 2, the fan 12 drives an incoming airflow F which is then separated into a primary airflow Fl and a secondary airflow F2. The primary airflow Fl passes through a primary body 21 of the turbomachine and the secondary airflow F2 bypasses the primary body 21. The primary body 21 is delimited by a nacelle 26. The primary airflow Fl then passes through the primary body 18 via an internal circulation channel 20

[0034] The primary flow Fl and the secondary airflow FII both contribute to the propulsion of the aircraft.

[0035] In the following, the terms "upstream" and "downstream" are understood to refer to the direction of flow of an airflow through the internal circulation channel 20.

[0036] The primary airflow Fl passes successively through different elements of the primary body 21, notably a low-pressure compressor 22 and then a high-pressure compressor 24 which supplies compressed air to a combustion chamber 34. A high-pressure turbine 36 is located downstream of the combustion chamber 34 and receives the exhaust gases exiting the latter. The gases are then transmitted to a low-pressure turbine 38, and are then expelled through an exhaust nozzle 40, thus generating thrust to propel the aircraft.

[0037] The high-pressure turbine 36 is coupled to the high-pressure compressor 24 via a high-pressure transmission shaft 42 which allows the high-pressure compressor 32 to be driven by the high-pressure turbine 36. Similarly, the low-pressure turbine 38 is coupled to the low-pressure compressor 22 via a low-pressure transmission shaft 54 ​​which allows the low-pressure compressor 22 to be driven by the low-pressure turbine 38.

[0038] Furthermore, the turbomachine 2 includes a coupling system (or transmission system) 50 by which the low-pressure turbine 36 drives the blower 12 in rotation around a longitudinal X axis of the turbomachine 2.

[0039] The coupling system 50 includes a reducer 52 for coupling an input shaft, which is the low pressure shaft 54, with an output shaft 56 which is the shaft driving the blower 12.

[0040] With reference to [Fig.3], the reducer 52 has an input pinion 58 which is driven in rotation by the input shaft 54 ​​and which surrounds at least part of the latter.

[0041] Preferably, the reducer 52 is an epicyclic gear reducer comprising a sun gear forming the input pinion 58, a ring gear surrounding the sun gear 58, and a plurality of planet gears, carried by a planet carrier, meshing with the sun gear 58 and the ring gear. One of the ring gear and the planet carrier is fixed, and the output shaft 56 is driven in rotation by the other of the ring gear and the planet carrier. Preferably, the gears of the reducer 52 are made with herringbone teeth.

[0042] As shown in [Fig. 4], the propulsion system 2 comprises an assembly 60 including a rotor 62 rotating about a longitudinal axis (X) relative to a fixed structure 64. The rotor 62 includes an upstream section 56 which is the shaft for driving the fan 12 (or fan shaft) in rotation about the axis X. The rotor 62 also includes a downstream section which is the low-pressure turbine shaft 54. The upstream section 66 and the downstream section 54 are connected by the transmission system which, in this embodiment, is the gearbox 52. Due to the unavoidable axial displacements of the rotor 62, the upstream section 56 and the downstream section 54 induce an axial force on the gearbox 52 during the operation of the turbomachine 2. This axial force includes, in particular, compressive and / or tensile forces which are applied to the gearbox by the rotor.

[0043] In order to limit these axial forces, a bearing is positioned on each of the upstream 56 and downstream 54 sections. Thus, an upstream bearing 70 and a downstream bearing 80 make it possible to connect the upstream 56 section and the downstream 54 section respectively to the fixed structure 64. In this embodiment, the upstream bearing 70 and the downstream bearing 80 are ball bearings, but the invention also applies to other types of bearings with axial force transfer (or axial thrust bearings), such as tapered roller bearings.

[0044] With reference to [Fig. 6a], the upstream bearing 70 comprises an upstream inner ring 72 and an upstream outer ring 74. The upstream inner ring 72 is integral with the rotor 62, in particular with the upstream section 56, and is thus rotatably mounted around the axis (X). The position of the upstream inner ring 72 around the upstream section 56 can be maintained by means of one or more ridges on the upstream section 56 forming one or more stops for the upstream inner ring 72. The upstream outer ring 74 is connected to the fixed structure 64 forming the stator. The mobility of the upstream inner ring 72 within the upstream outer ring 74 is enabled by means of a ball bearing 76.

[0045] The downstream bearing 80 differs from the upstream bearing 70 only in that it is connected to the downstream section 54, the references 82, 84 and 86 of the downstream bearing 80 correspond to the references 72, 74 and 76 of the upstream bearing 70.

[0046] The rotor 62 being axially held by the upstream bearing 70 and the downstream bearing 80, the upstream sections 56 and downstream sections 54 induce an axial force on the reducer 52 during the operation of the turbomachine 10. This axial force includes in particular compressive forces and / or tensile forces which are applied by the upstream section 56 and the downstream section 54 on the reducer 52.

[0047] This axial force can be limited by a mounting method 100 of the assembly 60 according to an embodiment of the invention and described with reference to [Fig.5].

[0048] The assembly process comprises the following steps: - SI: determine a maximum value of the axial force which must not be exceeded when the propulsion system 2 is in operation; - S2: Determine at least one target parameter from among: an axial positioning at rest of the inner ring 72 or 82 of one of the bearings 70 or 80 relative to its outer ring 74 or 76, and an axial stress value at rest to which the rotor 62 is subjected between the upstream bearing 70 and the downstream bearing 80 when the propulsion system 2 is at rest, so that, during the operation of the propulsion system 2, the axial force does not exceed the maximum value determined; - S3: configure at least one of the upstream bearing 70 and the downstream bearing 80 so as to obtain said axial positioning at rest of the inner ring 72 or 82 of the bearing 710 or 80 relative to its outer ring 74 or 84, and / or dimension the length of the rotor 62 between the two bearings 70 and 80 so that the rotor 62 is subjected to axial stress at rest once mounted between the two bearings 70 and 80 while the propulsion system 2 is at rest.

[0049] The axial force induced on the transmission system by the upstream 56 and downstream 54 sections of the rotor 62, during the operation of the propulsion system 2, is thus limited and does not exceed the maximum value determined.

[0050] The various parameters of the assembly that can be taken into account to determine the maximum value of the axial force during operation may be the type of bearings, the dimensions of the bearings and the rotor, and / or the type and dimensions of the transmission system. These parameters may also include the flexibility values ​​of the rotor and stator, the power transmitted through the gearbox, and / or the axial forces transmitted through the rotor during the operation of the propulsion system. The axial forces transmitted through the gearbox correspond in particular to the rotor displacements due to thermal expansion, which are prevented by the presence of the two bearings, and to the tensile or compressive forces generated by the blower, a compressor, and / or a turbine.

[0051] The maximum value of the axial force during operation can also be determined by taking into account the conditions to which the assembly 60 is subjected when the propulsion system 2 is in operation, such as temperature conditions, the surrounding fluid(s), or pressure. The axial force during operation corresponds in particular to a restrained displacement due to the presence of the upstream and downstream bearings. The relationship between the restrained displacement of the rotor and the resulting force is determined by the total axial flexibility of the rotor. The assembly method makes it possible to define the cold position of the rotor and / or to introduce a restrained axial displacement of the rotor at rest in order to obtain the desired value of the restrained axial displacement of the rotor during operation and thus the resulting force.

[0052] According to this embodiment, the parameters of assembly 60 and the conditions to which it is subjected during the operation of the turbomachine 10 are as follows: - a temperature greater than or equal to 50°C and less than or equal to 200°C; - surrounding fluids: mixture of air and oil in different proportions; - axial forces passing through the rotor line having a value greater than or equal to 0 and less than or equal to 10,000 Newton (N); these axial forces are evaluated by calculation and include in particular the tensile or compressive forces exerted by the blower and / or the low pressure shaft on the rotor, the contraction or elongation (expansion) of the stator under the effect of temperature variations, and also the contraction or elongation of the stator under the effect of temperature variations. - power passing through the reducer 52 having a value greater than or equal to 0 and less than or equal to 22,000 hp (horse power); this power corresponds in particular to the order of magnitude of the propulsive power of a short and medium-range twin-engine aircraft engine; - an ambient pressure greater than or equal to 0 and less than or equal to 5 bars; - rotor flexibility less than or equal to 107 m / N and greater than or equal to 108; - stator flexibility greater than or equal to 0 and less than or equal to 108 m / N.

[0053] The axial flexibility of the rotor and stator is estimated by calculation and corresponds to the sum of the axial flexibility of the parts that make up said stator and said rotor.

[0054] Once the maximum axial force value in operation has been determined in step S2, the determination of at least one of the target parameters makes it possible to carry out an optimized assembly in step S3 so that the maximum force value is not exceeded during operation.

[0055] During step S2, the configuration of at least one of the upstream bearing 70 or the downstream bearing 80 involves determining an axial position of at least one of the downstream inner ring (82) relative to the downstream outer ring (74) or of the upstream inner ring (72) relative to the upstream outer ring (84).

[0056] As shown in [Fig. 6a], the rings have a radial plane of symmetry R extending perpendicularly to the X-axis and intersecting each ring at its midpoint along an axial direction. The midpoint of the outer ring is referenced Ce, the midpoint of the inner ring is referenced Ci, and the center of the balls positioned between the two rings is referenced Co. It is thus possible to determine the axial position of one ring relative to the other by the position of the projections of the centers Ce and Ci onto the longitudinal axis.

[0057] When the projection of the center of the outer ring Ce, the center of the ring Ci, and the center of the balls Co onto the X-axis coincide, the outer ring 74 (84) and the inner ring 72 (82) are considered to be axially aligned, as illustrated in [Fig. 6a]. When the outer ring 74 (84) and the inner ring 72 (82) are axially aligned, an upstream clearance Jam (between an upstream wall of the ball 76 and an internal portion of the arc formed by each ring on the upstream side) and a downstream clearance Jav (between a downstream wall of the ball 76 and an internal portion of the arc formed by each ring on the downstream side) have the same value, which is half the value of the total clearance between the ball and the arc formed by each of the rings.

[0058] According to one possible embodiment, the total clearance value can be on the order of 0.2 mm, i.e., two half-clearances Jav and Jam of 0.1 mm each.

[0059] When the projection of the center of the outer ring Ce, the center of the ring Ci, and the center of the balls Co onto the X-axis do not coincide, the outer ring 74 (84) and the inner ring 72 (82) are considered to be axially offset, as illustrated in Figures 6b and 6c. The axial offset can be partial when the inner ring 72 (82) and the outer ring 74 (84) are offset, but neither the upstream clearance Jam nor the downstream clearance Jav is zero. The offset can also be total when one of the upstream clearance Jam and the downstream clearance Jav is zero; in this case, the inner ring 72 (82) and the outer ring 74 (84) abut against the balls 76, one on the upstream side and the other on the downstream side.

[0060] When the bearing 70 (or 80) is in the offset position of [Fig.6b] (upstream offset), the rotor 62 can move downstream but is blocked in the upstream direction since the inner ring 72 (or 82) comes into contact with the balls 76 (or 86) in the upstream direction.

[0061] When the bearing 70 (or 80) is in the offset position of [Fig.6c] (downstream offset), the rotor 62 can move upstream but is blocked in the downstream direction since the inner ring 72 (or 82) comes into contact with the balls 76 (or 86) in the downstream direction.

[0062] Determining the position of the upstream inner ring 72 relative to the upstream outer ring 74 and / or the downstream inner ring 82 relative to the downstream outer ring 84 allows the value of the clearance at the upstream and / or downstream bearing to be chosen, the value of the clearance determining the possibility of axial displacement of the inner ring relative to the outer ring and the amplitude of this displacement.

[0063] According to one possible embodiment, the inner ring of one of the bearings can be positioned axially offset relative to the corresponding outer ring, and the other inner ring can be positioned axially aligned with the corresponding outer ring. According to this embodiment, one of the bearings has maximum axial clearance and allows the rotor a slight displacement, which may be upstream or downstream, while the other bearing allows less, or even no, displacement in either the upstream or downstream direction, depending on whether the axial offset is upstream or downstream, respectively.

[0064] Referring to [Fig.7], the positioning of the inner ring of one of the bearings, for example the inner ring 72 of the upstream bearing 70, in a manner axially offset relative to the upstream outer ring 74 may involve the placement of a shim 90 between the inner ring 72 and a relief 92 of the rotor 66. The shim 90 forming a thickness between the relief 92 and the inner ring 72, it allows the upstream inner ring 72 to be axially offset relative to the upstream outer ring 74 in the downstream direction.

[0065] The use of the 90 mm shim allows the offset value to be selected, which can be partial or total. The 90 mm shim is obtained from a nominal shim that is machined to give it the desired axial thickness. The 90 mm shim can have a thickness along the axial direction of up to 0.1 mm to obtain a total offset and, for example, 0.05 mm to obtain a partial offset.

[0066] Fig. 8a represents a variant in which the rotor 62 is positioned between the upstream bearing 70 and the downstream bearing 80. In this variant, the upstream inner ring 72 and the upstream outer ring 74 are axially aligned, and the downstream inner ring 82 is offset upstream relative to the downstream outer ring 84.

[0067] In [Fig.8b], the addition of the shim 90 against the downstream inner ring 82, on its upstream side, allows the downstream inner ring 82 to be axially aligned with the downstream outer ring 84.

[0068] In [Fig.8c], the addition of a thicker shim 90' along the axial direction relative to the shim 90 of [Fig.8b] allows the downstream bearing 80 to be placed in an offset position, the downstream inner ring 82 being offset downstream relative to the downstream outer ring 84.

[0069] The use of a shim thus makes it possible, starting from a given length of the rotor 62, to adjust the desired axial alignment of the inner and outer rings of one of the bearings, so that the rotor is subjected to axial stress at rest once mounted between the two bearings while the propulsion system is stopped. Adding a shim can therefore allow the desired offset to be adjusted for at least one of the bearings and also to determine the rotor length.

[0070] According to one possible variant, it is also possible to dimension the length of the rotor between the two bearings by applying a thermal differential between the rotor 62 and the stator 64 at the time of positioning the rotor 62 between the upstream bearing 70 and the downstream bearing 80, then stabilizing the assembly 60 at the same temperature in a rest position.

[0071] According to this variant shown in figures 9b and 9c, a final temperature Tf of the assembly 60 at rest is considered, after mounting the rotor 62 between the upstream bearing 70 and the downstream bearing 70.

[0072] With reference to Figure 9a, if the assembly is carried out at temperature Tf and therefore without subjecting the rotor 62 to a thermal differential before assembly, we obtain an assembly 60 according to the prior art in which the rotor 62 is positioned between the upstream bearing 70 and the downstream bearing 80, the two bearings having their rings axially aligned, and the rotor 62 is then not under stress.

[0073] According to one embodiment of the invention, the rotor 62 can be subjected, before and during assembly, to an intermediate temperature Tint higher than the final temperature Tf. At the intermediate temperature Tint, the rotor 62 is expanded and therefore has an intermediate length greater than its length at the final temperature Tf, and also greater than its initial length before application of the thermal differential. The rotor 62 is positioned between the upstream and downstream bearings 70 and 80 at its intermediate length, and then the assembly stabilizes at the final temperature Tf. The rotor 62, under the effect of the temperature decrease, contracts. As the rotor 62 is fixed between the upstream and downstream bearings 70 and 80, its length tends to shorten by exerting a tensile force on the upstream and downstream bearings. In the example of [Fig.[9b], the upstream bearing has no axial mounting clearance, and the tensile force is therefore exerted solely on the downstream bearing 80, which stabilizes in an axially offset position, its downstream inner ring 82 being in an upstream position relative to its downstream outer ring 84, as shown in [Fig. 9b]. According to another embodiment, both the upstream and downstream bearings may have axial clearance, and the tensile force is then exerted equally on both the upstream and downstream bearings, which then both position themselves in an offset position.

[0074] According to another embodiment, the rotor 62 can be subjected, before and during assembly, to an intermediate temperature Tint lower than the final temperature Tf. At the intermediate temperature Tint, the rotor 62 is then contracted and therefore has an intermediate length shorter than its length at the final temperature Tf, and also shorter than its initial length before application of the thermal differential. The rotor 62 is positioned between the upstream and downstream bearings 70 and 80 at its intermediate length, and then the assembly stabilizes at the final temperature Tf. The rotor 62, under the effect of the temperature rise, expands. As the rotor 62 is fixed between the upstream and downstream bearings 70 and 80, its length tends to elongate by exerting a force of Thrust on the upstream bearing and on the downstream bearing. In the example of [Fig. 9c], the upstream bearing has no axial mounting clearance, and the thrust force is therefore exerted solely on the downstream bearing 80, which stabilizes in an axially offset position, its downstream inner ring 82 being in the downstream position relative to its downstream outer ring 84, as shown in [Fig. 9c]. According to another embodiment, both the upstream and downstream bearings may have axial clearance, and the thrust force is then exerted equally on both the upstream and downstream bearings, which then both position themselves in an offset position.

[0075] According to one possible embodiment, when a thermal differential is applied to the rotor, a first part of the two bearings can be assembled with the stator, then the thermal differential is applied to the rotor, and finally the second part of the two bearings is fixed to the rotor so that the two bearings can be assembled. The rotor then returns to the final temperature by exerting a stress (tension or thrust) on the bearings. According to another possible embodiment, it is possible to assemble the first part of the bearings first with the rotor and then assemble the second part with the stator.

[0076] Applying a thermal differential to the rotor allows the rotor to be subjected to axial stress at rest once mounted between the two bearings, while the propulsion system is stopped. In this way, during operation of the propulsion system 2, the axial force will not exceed the determined maximum value.

Claims

Claims

1. Method for mounting (100) an assembly (60) of a propulsion system (2) for an aircraft, said assembly (60) comprising a rotor (62) rotating about a longitudinal axis (X) relative to a fixed structure (64), said rotor (62) comprising an upstream section (56) and a downstream section (54) connected to each other by a transmission system (50), the upstream and downstream sections inducing an axial force on the transmission system during operation of the propulsion system (2), the upstream section (56) being connected to the fixed structure (64) by an upstream bearing (70) and the downstream section (54) being connected to the fixed structure (64) by a downstream bearing (80), the upstream bearing (70) comprising an upstream inner ring (72) and an upstream outer ring (74), the downstream bearing (80) comprising a downstream inner ring (82) and a downstream outer ring (84),said method (100) comprising the following steps: - S1: determining a maximum value of the axial force that must not be exceeded when the propulsion system (2) is in operation; - S2: determining at least one target parameter from among: a) an axial positioning at rest of the inner ring (72, 82) of one of the bearings (70, 80) relative to its outer ring (74, 84), and b) an axial stress value at rest to which the rotor (62) is subjected between the upstream bearing (70) and the downstream bearing (80) when the propulsion system (2) is stopped, so that, during operation of the propulsion system, the axial force does not exceed the determined maximum value; - S3: configure at least one of the upstream bearing (70) and the downstream bearing (80) so as to obtain said axial positioning at rest of the inner ring (72, 82) of the bearing (70, 80) relative to its outer ring (74, 84), and / or dimension the length of the rotor (62) between the two bearings (70,80) so that the rotor (62) is subjected to the axial stress at rest once mounted between the two bearings while the propulsion system (2) is stopped.,

2. The method of claim 1, wherein configuring at least one of the upstream bearing (70) and the downstream bearing (80) comprises determining an axial position of at least one of the downstream inner ring (82) relative to the downstream outer ring (84) and the upstream inner ring (72) relative to the upstream outer ring (82).

3. A method according to claim 1 or 2, comprising positioning the inner ring (72, 82) of one of the bearings (70, 80) axially offset relative to the corresponding outer ring (74, 84), and positioning the other inner ring (72, 82) axially aligned relative to the corresponding outer ring (82, 84).

4. Method according to claim 3, in which the positioning of the inner ring (72, 74) of one of the bearings (70, 80) in an axially offset manner relative to the corresponding outer ring (82, 84) comprises the placement of a shim (90) between the inner ring and a relief of the rotor (62).

5. A method according to any preceding claim, wherein dimensioning the length of the rotor (62) between the two bearings (70, 80) comprises applying a thermal differential between the rotor (62) and the stator at the time of positioning the rotor (62) between the two bearings (70, 80), then stabilizing the assembly (60) at the same temperature in a rest position.

6. A method according to claim 5, wherein applying a thermal differential between the rotor (62) and the stator comprises applying to the rotor (62) a temperature above the stabilization temperature, such that the rotor (62) is expanded, and then positioning the rotor (62) between the two bearings (70, 80) when in this expanded state.

7. A method according to claim 5, wherein applying a thermal differential between the rotor (62) and the stator comprises applying to the rotor (62) a temperature lower than the stabilization temperature, such that the rotor (62) is contracted, and then positioning the rotor (62) between the two bearings (70, 80) when in this contracted state.

8. Method according to any one of the preceding claims, in which the upstream bearing (70) and the downstream bearing (80) are bearings capable of taking up axial forces.

9. Assembly (60) of propulsion system (2) for aircraft, said assembly (60) comprising a rotor (62) rotating about a longitudinal axis (X) relative to a fixed structure (64), said rotor (62) comprising an upstream section (56) and a downstream section (54) connected to each other by a transmission system (50), the upstream section (56) being connected to the fixed structure (64) by an upstream bearing (70) and the downstream section (80) being connected to the fixed structure (64) by a downstream bearing (80), the upstream bearing (70) comprising an upstream inner ring (72) and an upstream outer ring

10.

11.

12. (82), the downstream bearing (80) comprising a downstream inner ring (82) and a downstream outer ring (84), wherein the inner ring of one of the bearings is axially offset relative to the corresponding outer ring, the inner ring of the other bearing being axially aligned relative to the corresponding outer ring. An aircraft propulsion system assembly (60) according to claim 9, wherein the rotor (62) is under axial stress so that its rest length between the upstream (70) and downstream (80) bearings, when the rotor (62) is positioned between these two bearings once assembly is complete and the propulsion system (2) is stationary, is different from an initial length when it is not under stress. An aircraft propulsion system comprising an assembly (60) according to claim 9 or 10 or an assembly (60) obtained by means of a method (100) according to any one of claims 1 to 8. Aircraft (1) comprising at least one propulsion system (2) according to claim 11.

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