Mechanical power transmission system, power transmission box and aircraft

Abstract

The transmission system (10) according to the invention comprises a driving shaft (1), a driven shaft (2), and a linkage device (3) enabling said driving shaft (1) to transmit a mechanical torque to said driven shaft (2), as well as guiding devices (5, 6, 7, 8) guiding said driving shaft (1) in rotation about a driving axis (AXMA) and said driven shaft (2) about a driven axis (AXME). When said transmission system (10) is at rest, said driving axis (AXMA) and said driven axis (AXME) are misaligned, and move closer to each other when said transmission system (10) transmits a torque from said driving shaft (1) to said driven shaft (2). (Shorthand figure: Figure 1)

Description

Title of the invention: Mechanical power transmission system, power transmission box and aircraft

[0001] The present invention lies in the field of power transmission devices.

[0002] The present invention relates to a transmission system, a power transmission box and an aircraft, as well as a method for limiting parasitic forces in such a transmission system. In particular, a transmission system may have the function of mechanically transmitting power between an engine and a rotor of an aircraft.

[0003] A known transmission system comprises a connecting device transferring a rotational movement as well as a mechanical torque from a driving shaft to a driven shaft. In addition, the transmission system may comprise one or more guide devices for guiding the driving shaft and the driven shaft in rotation relative to a support. Such guide devices may comprise, for example, ball or roller bearings. Furthermore, the driving shaft and the driven shaft are aligned and coaxial.

[0004] However, under certain operating conditions of the transmission system, the driving shaft and the driven shaft may be subjected to significant radial or even axial loads capable of causing deformations and displacements generating radial and / or angular misalignment between them. Such misalignment may tend to generate parasitic forces in the power transmission system capable of creating wear and / or damage to the connecting device and / or the guide devices, or even to the driving shaft and / or the driven shaft.

[0005] To at least limit the transmission of parasitic forces between the driving and driven shafts, various solutions have been developed.

[0006] For example, document FR 3057850 describes a transmission system comprising a first and a second shaft secured respectively to two coupling means. This transmission system also comprises a connection system, called "flexible", comprising at least one intermediate shaft connecting the two coupling means, and passing longitudinally through the second shaft.

[0007] In addition, the length of the intermediate shaft can be maximized, in order to minimize the effect of a possible misalignment between the first shaft and the second shaft. Indeed, the longer the intermediate shaft, the less the parasitic forces generated by a given misalignment between the first shaft and the second shaft are significant.

[0008] In a complementary or alternative manner, the intermediate shaft may have a intrinsic flexibility allowing misalignments between the first shaft and the second shaft to be tolerated so as to limit the parasitic forces generated.

[0009] Furthermore, according to this document FR 3057850, the connecting device may comprise particular coupling means to minimize the impact of axial, radial and angular misalignments. These coupling means may for example take the form of diaphragms or plate systems.

[0010] According to another example, document US 5286117 describes a bearing whose outer ring is asymmetrical so that its mounting in a cylindrical housing and around a shaft allows the presence of radial or axial clearance between this outer ring and the housing, only over a part of the circumference of this outer ring. Consequently, this bearing makes it possible to reduce the stresses caused by a misalignment between the inner and outer rings of the bearing in order to limit the load peaks on the rolling elements of the bearing, as well as the other parts of the system.

[0011] Alternative solutions exist. For example, document FR 2988789 describes a device for axially wedging a bearing assembled on a shaft. This axial wedging device comprises an angular wedging means and an operating clearance allowing limited angular movement of the bearing.

[0012] According to one technology, the housing or casing of a transmission system can be very rigid, to limit the amplitudes of misalignment of the shafts. Although effective, this solution has a cost in terms of mass.

[0013] The present invention therefore aims to propose an alternative power transmission system capable of minimizing the transmission of parasitic forces, in particular between a driving shaft and a driven shaft of the transmission system during its operation.

[0014] According to the invention, a transmission system comprises a driving shaft, a driven shaft and a connecting device mechanically connecting the driving shaft and the driven shaft, the connecting device allowing the driving shaft to rotate the driven shaft. In this way, the connecting device makes it possible to transmit a rotational torque at least from the driving shaft to the driven shaft, or even also from the driven shaft to the driving shaft.

[0015] The transmission system also comprises at least two first housings and at least two first guide devices for guiding the driving shaft in rotation around a driving axis AXMA, each first housing receiving one of the first guide devices. The transmission system also comprises at least two second housings and at least two second guide devices for guiding the driven shaft in rotation around a driven axis AXME, each second housing receiving one of the second guide devices.

[0016] The transmission system may also comprise a casing in which the first and second housings are formed. This casing may be a single piece or composed of several assembled sub-assemblies.

[0017] The transmission system according to the invention is notably remarkable in that the driving axis AXMA and the driven axis AXME are misaligned when the transmission system is stopped, the driving shaft as well as the driven shaft and the connecting device being located around a center line LM of the driving axis AXMA and the driven axis AXME.

[0018] This center line LM is defined by being, in each plane perpendicular to this center line LM, equidistant from the driving axis AXMA and the driven axis AXME. The driving shaft, the driven shaft and the connecting device are located around this center line LM, and are thus in the extension of each other, the connecting device being located between the driving shaft and the driven shaft.

[0019] The driving AXMA and driven AXME axes are thus close to each other, but are not coaxial when the transmission system is stopped, namely when no mechanical torque is transmitted between the driving shaft and the driven shaft. A gap then separates the driving AXMA axis and the driven AXME axis.

[0020] In this way, when the transmission system transmits a mechanical torque, in particular from the driving shaft to the driven shaft, forces, also referred to as "loads", lead to a bringing together of the driving axis AXMA and the driven axis AXME by a reduction in the gap separating them. This bringing together of the driving axis AXMA and driven axis AXME can go in particular until the driving axis AXMA and the driven axis AXME are substantially aligned and coaxial under predetermined conditions.

[0021] The initial misalignment between the AXMA driving axis and the AXME driven axis is thus intended to anticipate and compensate for deformations and / or displacements under load, during operation of the transmission system.

[0022] The bringing together of the driving AXMA and driven AXME axes makes it possible, during operation, to limit, or even avoid, the appearance of parasitic forces in the transmission system, or in particular at the level of the driving and driven shafts. The reduction or even the absence of such parasitic forces also makes it possible to limit the forces undergone by the connecting device and the guiding devices of the transmission system, as well as by any speed reduction devices likely to be located upstream of the driving shaft and downstream of the driven shaft.

[0023] The reduction, or even the absence, of such parasitic forces advantageously makes it possible to improve the operation of the transmission system and to increase its service life, thereby reducing the frequency of maintenance operations and / or the replacement of components.

[0024] The transmission system according to the invention can also make it possible to obtain a weight saving compared to solutions aimed at stiffening the transmission system.

[0025] Surprisingly, the misalignment at rest of the AXMA driving shaft and AXME driven shaft is compatible with the operation of the transmission system, in particular during its start-up or when a low mechanical torque is transmitted by the driving shaft to the driven shaft. Indeed, the connecting device as well as any speed reduction devices accept such misalignment under these conditions without their operation being impaired.

[0026] The method according to the invention may comprise one or more of the following characteristics, taken alone or in combination.

[0027] According to one example, the connecting device comprises at least one drive device allowing the driving shaft to transmit a mechanical torque to the driven shaft.

[0028] The connecting device and / or such a driving device may not comprise any speed reduction device, so that the driving shaft and the driven shaft rotate at substantially the same speed when the driving shaft transmits a mechanical torque to the driven shaft.

[0029] Furthermore, said at least one drive device may be permanent, and similarly provide mechanical torque from the driving shaft to the driven shaft, or from the driven shaft to the driving shaft. Alternatively, said at least one drive device may be non-permanent, and in particular provide motor torque only from the driving shaft to the driven shaft.

[0030] Said at least one drive device may, for example, comprise a device chosen from a list comprising splines, at least one key, a bolted assembly, a freewheel, a clutch, a diaphragm coupling means.

[0031] The connecting device may comprise an intermediate shaft, said at least one drive device comprising a first drive device and a second drive device, the first drive device transmitting a mechanical torque at least from the driving shaft to the intermediate shaft and the second drive device transmitting a mechanical torque at least from the intermediate shaft to the driven shaft. In this case, the driving shaft, the intermediate shaft and the driven shaft may be substantially aligned and coaxial when the driving AXMA and driven AXME axes move under load during operation of the transmission system.

[0032] According to another example compatible with the previous ones, the driving axis AXMA and the driven axis AXME can be parallel to each other when the transmission system is stopped.

[0033] Alternatively, the driving axis AXMA and the driven axis AXME may be inclined relative to each other, and therefore not parallel to each other, when the transmission system is stopped. The distance separating the driving axis AXMA and the driven axis AXME is in this case an angular distance.

[0034] According to another example compatible with the previous ones, each first guide device, arranged in a first housing, comprises a first center of rotation, around which a first inner ring of the first guide device rotates, relative to a first outer ring of the first guide device. Similarly, each second guide device, arranged in a second housing, comprises a second center of rotation, around which a second inner ring of the second guide device rotates, relative to a second outer ring of the second guide device. The first centers of rotation and the second centers of rotation are, for example, located respectively on planes of symmetry of the first and second guide devices.

[0035] The driving axis AXMA then passes through the first centers of rotation, and the driven axis AXME passes through the second centers of rotation. The driving axis AXMA is thus defined by the first centers of rotation and the driven axis AXME is defined by the second centers of rotation.

[0036] The misalignment value between the driving axis AXMA and the driven axis AXME is then defined by the relative positions of the first and second centers of rotation.

[0037] In particular, if the driving axis AXMA and the driven axis AXME are parallel to each other when the transmission system is stopped, the relative positions of the first centers of rotation respectively with respect to the second centers of rotation are identical.

[0038] Alternatively, if the driving axis AXMA and the driven axis AXME are not parallel to each other when the transmission system is stopped, the relative positions of the first centers of rotation respectively with respect to the second centers of rotation are different.

[0039] According to a first variant of the transmission system according to the invention, the first housings are cylindrical and centered on the driving axis AXMA, and the second housings are cylindrical, and are centered on the driven axis AXME when the transmission system is stopped. In this case, the first housings and the second housings are not coaxial.

[0040] The misalignment between the driving axis AXMA and the driven axis AXME is thus obtained directly from the relative positions of the first housings and the second housings. In this case, the first guide devices and the second guide devices conventionally comprise coaxial inner and outer rings.

[0041] According to a second variant of the transmission system according to the invention, the first housings and the second housings are cylindrical, and are coaxial when the transmission system is stopped, and at least one of the first guide devices and the second guide devices comprises an eccentric outer ring.

[0042] This eccentric outer ring comprises an outer peripheral surface and an inner peripheral surface which are eccentric relative to each other and therefore not coaxial. This eccentric outer ring thus makes it possible to move the center of rotation of the guide device concerned relative to the center of the housing in which the guide device concerned is arranged. On the other hand, the inner ring of a guide device is coaxial with the inner peripheral surface of the associated eccentric outer ring. Consequently, the inner ring of this guide device and the housing in which this guide device is arranged are not coaxial.

[0043] The misalignment between the driving axis AXMA and the driven axis AXME is thus obtained by means of the eccentric outer ring of at least one of the first guide devices and the second guide devices. For example, the first two guide devices each comprise a first eccentric outer ring, the second guide devices comprising second coaxial inner and outer rings.

[0044] According to a third variant of the transmission system according to the invention, the first housings and the second housings are cylindrical, and are coaxial when the transmission system is stopped, and at least one eccentric ring is inserted between on the one hand one of the first housings and second housings, and on the other hand one of the first guide devices and the second guide devices.

[0045] This eccentric ring comprises an outer peripheral surface and an inner peripheral surface which are eccentric with respect to each other and therefore not coaxial. In this way, this eccentric ring makes it possible to move the center of rotation of the guide device around which the eccentric ring is inserted, relative to the center of the housing in which the eccentric ring is inserted. Consequently, the guide device and the housing in which this guide device is arranged are not coaxial.

[0046] The misalignment between the driving axis AXMA and the driven axis AXME is thus obtained by means of the eccentric ring(s). For example, two eccentric rings are inserted respectively between the first housings and the first two guide devices.

[0047] According to a fourth variant of the transmission system according to the invention, the transmission system comprises a casing composed of several assembled sub-assemblies including at least a first sub-assembly and a second sub-assembly, the first sub-assembly comprising said at least two first housings and the second sub-assembly comprising said at least two second housings.

[0048] The first subassembly and the second subassembly are fixed to each other such that the driving axis AXMA and the driven axis AXME are misaligned when the transmission system is stopped.

[0049] The first subassembly and the second subassembly are thus offset relative to each other, relative to a position in which the driving axis AXMA and the driven axis AXME would be aligned when the transmission system is stopped. In this case, the first subassembly and the second subassembly are also offset relative to each other, relative to a position in which the first housings and the second housings are coaxial when the transmission system is stopped, the first housings and the second housings being cylindrical.

[0050] The misalignment between the driving axis AXMA and the driven axis AXME is thus obtained by means of this offset between the first subassembly and the second subassembly. This offset can be obtained by the relative positions of the connection points between the first subassembly and the second subassembly, or using an intermediate part adapted to the desired offset.

[0051] The first subassembly and the second subassembly may be fixed directly to each other, or via the intermediate part, for example a third subassembly of the casing.

[0052] The present invention also relates to a power transmission box comprising a transmission system as previously described.

[0053] The power transmission box may, for example, be positioned between an engine and a propulsion device, such as a rotor if the power transmission box is fitted to an aircraft.

[0054] The present invention also relates to an aircraft comprising a transmission system as previously described, arranged or not in a power transmission box.

[0055] The present invention finally aims at a method of limiting parasitic forces in a transmission system as previously described.

[0056] This method includes in particular the following steps: - determination of a relative displacement between the driving axis AXMA and the driven axis AXME during operation of the transmission system, and - misalignment, when the transmission system is stopped, of the AXMA driving axis and the AXME driven axis depending on this relative displacement.

[0057] This method thus makes it possible to determine the value of the misalignment to be applied between the driving axis AXMA and the driven axis AXME at rest. The value of this misalignment alignment can be determined based on the relative displacement between the AXMA driving axis and the AXME driven axis, itself determined by simulation and possibly confirmed by tests, or determined directly by tests.

[0058] During the step of determining a relative displacement between the driving axis AXMA and the driven axis AXME, the relative displacement between the driving axis AXMA and the driven axis AXME may be, for example, determined when the transmission system transmits a mechanical torque from the driving shaft to the driven shaft greater than or equal to a predetermined torque. This predetermined torque may, for example, correspond to a particular operating phase of the transmission system, for example the operating phase that is most penalizing in terms of forces experienced by the transmission system, or the most used.

[0059] The relative displacement between the driving axis AXMA and the driven axis AXME may alternatively be determined as being equal to an average relative displacement of the driving axis AXMA and the driven axis AXME when the transmission system transmits a mechanical torque from the driving shaft to the driven shaft which is included in a predetermined interval. This predetermined interval may, for example, correspond to one or more operating phases of the transmission system, in particular the operating phases which are the most penalizing in terms of forces undergone and / or the most used.

[0060] The relative displacement between the driving axis AXMA and the driven axis AXME can be determined as being equal to an average relative displacement of the driving axis AXMA and driven axis AXME when the transmission system is in one or more predetermined operating phases, for example the operating phase or phases that are the most penalizing in terms of forces undergone or the most used.

[0061] When the transmission system according to the invention equips a rotary wing aircraft, for example by being positioned between an engine and a rotor of the aircraft, the most penalizing flight phases of the aircraft may be the takeoff, cruising flight, or even hovering flight phases, the most used flight phase generally being cruising flight.

[0062] However, this relative displacement between the driving axis AXMA and the driven axis AXME under load can prove complex to measure directly. In this case, during tests and / or simulation, parasitic forces, such as a moment and / or a shear force, can be measured in certain parts of the transmission system, then the relative positions of the driving axis AXMA and driven axis AXME at standstill are modified by iteration to minimize these parasitic forces under load.

[0063] The invention and its advantages will appear in more detail in the context of the description which follows with examples given for illustrative purposes with reference to the appended figures which represent: - [Fig.l], a view of a transmission system according to the invention integrated in a power transmission box, - [Fig.2], a schematic view of a transmission system according to the invention, - [Fig. 3], a schematic view comparing a transmission system according to the invention when stationary and in operation, - [Fig.4], a partial view of a transmission system according to the invention, - [Fig.5], a partial view of a transmission system according to the invention, and - [Fig.6], a view of an aircraft equipped with a transmission system according to the invention.

[0064] Elements present in several distinct figures are assigned a single reference.

[0065] [Fig.l] represents a transmission system 10 intended to transmit a torque and a mechanical power between a driving shaft 1 and a driven shaft 2. This transmission system 10 can for example be arranged between a motor and a propulsion device. This transmission system 10 can for example be integrated in a power transmission box 50 partially visible in [Fig.l].

[0066] Whatever its arrangement, the transmission system 10 comprises a driving shaft 1, a driven shaft 2 and a connecting device 3 mechanically connecting the driving shaft 1 and the driven shaft 2. The connecting device 3 secures the driving shaft 1 and the driven shaft 2 at least in one direction of rotation so that the driving shaft 1 drives the driven shaft 2 in rotation at least in this direction by transmitting a mechanical torque to it. For this purpose, the connecting device 3 comprises at least one drive device 31, 32.

[0067] A drive device 31, 32 may, for example, comprise a freewheel for transmitting a mechanical torque from the driving shaft 1 to the driven shaft 2 in a single direction of rotation. Alternatively, a drive device 31, 32 may comprise splines arranged in a complementary manner on the driving shaft 1 and driven shaft 2, at least one key, or even a bolted assembly for transmitting a mechanical torque from the driving shaft 1 to the driven shaft 2 in both directions of rotation. A drive device 31, 32 may also comprise a clutch.

[0068] The connecting device 3 may also comprise, according to the example of transmission system 10 of [Fig. 1], an intermediate shaft 35, a first drive device 31 and a second drive device 32. The first drive device 31 transmits a mechanical torque from the driving shaft 1 to the intermediate shaft 35 and the second drive device 32 transmits a mechanical torque from the intermediate shaft 35 to the driven shaft 2. According to the example of transmission system transmission 10, the first drive device 31 comprises splines and the second drive device 32 comprises a freewheel.

[0069] The transmission system 10 also comprises at least two first housings 15, 16 and at least two respective first guide devices 5, 6 for guiding the driving shaft 1 in rotation around a driving axis AXMA. Each first housing 15, 16 receives one of the first guide devices 5, 6.

[0070] In addition, the transmission system 10 comprises at least two second housings 17, 18 and at least two respective second guide devices 7, 8 making it possible to guide the driven shaft 2 in rotation around a driven axis AXME relative to a casing 54 of the power transmission box 50. Each second housing 17, 18 receives one of the second guide devices 7, 8.

[0071] The first and second housings 15-18 may be cylindrical in shape and arranged / formed in the casing 54 of the power transmission box 50. This casing 54 may comprise several sub-assemblies 56-59, as shown in [Fig.l], these sub-assemblies 56-59 being able to be assembled together using screws for example.

[0072] Thus, according to the example of transmission system 10 shown in [Fig.l], the transmission system 10 comprises two first housings 15, 16, arranged respectively in two sub-assemblies 56, 57, and two second housings 17, 18, arranged respectively in two sub-assemblies 58, 59.

[0073] Furthermore, the guide devices 5-8 comprise, for example, bearings comprising an inner ring 52, 62, 72, 82 and an outer ring 53, 63, 73, 83 framing rolling elements, namely balls or rollers for example.

[0074] Each first guide device 5, 6 comprises a first center of rotation, around which the driving shaft 1 is guided in rotation. The driving axis AXMA thus passes through the first centers of rotation. Similarly, each second guide device 7, 8 comprises a second center of rotation, around which the driven shaft 2 is guided in rotation. The driven axis AXME thus passes through the second centers of rotation. The driving axis AXMA is thus defined by the positions of the first centers of rotation and the driven axis AXME is defined by the positions of the second centers of rotation.

[0075] The driving shaft 1 and the driven shaft 2 may also comprise teeth 14, 24 in order to cooperate respectively with an input shaft 51 and an output shaft 61 of the power transmission box 50.

[0076] According to one example, the driving shaft 1 can carry the teeth 14, the first guide devices 5, 6 guiding the driven shaft 2.

[0077] According to the example of transmission system 10 of [Fig.l], the driving shaft 1 may comprise a driving toothed wheel 141 provided with teeth 14. This driving toothed wheel 141 is secured in rotation to the driving shaft 1 by a device additional drive 12. The driving gear wheel 141 is guided in rotation directly or indirectly by the first guide devices 5, 6. Optionally, the first guide devices 5, 6 guide the drive shaft 1 by guiding the driving gear wheel 141. This driving gear wheel 141 cooperates with an input gear wheel 514 secured to the input shaft 51. In this way, the drive shaft 1 can be rotated for example by a motor, via the input shaft 51 and the input gear wheels 514 and driving 141.

[0078] Similarly, the driven shaft 2 may comprise a driven toothed wheel 241 provided with teeth 24 cooperating with an output toothed wheel 614 integral in rotation with the output shaft 61. The driven toothed wheel 241 is guided in rotation directly or indirectly by the second guide devices 7, 8. According to the example of [Fig.l], the driven shaft 2 and the driven toothed wheel 241 form a single piece.

[0079] In this way, when the driving shaft 1 transmits a rotating mechanical torque to the driven shaft 2, this mechanical torque is transmitted to the output shaft 61, possibly with a reduction depending on the number of teeth 24 and the number of teeth of the output toothed wheel 614. A system of the driving shaft wheel type is conceivable.

[0080] Regardless of these aspects, the driving AXMA and driven AXME axes are misaligned, and therefore are not coaxial, when the transition system 10 is stopped. Indeed, a gap separates the driving AXMA axis and the driven AXME axis when the transmission system 10 is stopped or not transmitting any torque from the driving shaft 1 to the driven shaft 2. Furthermore, the driving shaft 1 as well as the driven shaft 2 and the connecting device 3 are located around a center line LM which is in a middle position between the driving AXMA axis and the driven AXME axis.

[0081] This deviation can take the form of a distance e, as shown in [Fig.l], the driving axis AXMA and the driven axis AXME being parallel to each other. This deviation can take the form of an angle a, as shown in [Fig.2], the driving axis AXMA and the driven axis AXME then being inclined relative to each other, and therefore not parallel to each other. This [Fig.2] schematically represents only the driving shaft 1, the driven shaft 2 and the connecting device 3 as well as the driving toothed wheel 141. Such a misalignment between the driving axis AXMA and the driven axis AXME can also be defined simultaneously by a distance e and an angle a.

[0082] In Figures 1 and 2, the distance between the driving AXMA and driven AXME axes has been deliberately exaggerated to be clearly visible. The distance e is for example between 0.1 and 1 millimeter (0.1 and 1mm). The angle a is for example between 0.1 and 1 degrees (0.1 and 1°).

[0083] The value of this misalignment between the driving axis AXMA and the driven axis AXME, whether expressed in distance and / or in angle, does not prohibit the operation of the transmission system 10, in particular when starting it up or when a low torque is transmitted by the driving shaft 1 to the driven shaft 2. Indeed, the connecting device 3 as well as the input 514 and output 524 toothed wheels allow such misalignment of the driving AXMA and driven AXME axes, without generating a significant parasitic force as long as the transmitted torques remain low.

[0084] Advantageously, as soon as the torque transmitted by the transmission system 10, and in particular from the driving shaft 1 to the driven shaft 2, increases, deformations and / or displacements occur in the transmission system 10 according to the invention, in particular at the level of the driving shaft 1 and / or the driven shaft 2. These deformations and / or displacements tend to move / deform the driving shaft 1 and / or the driven shaft so as to reduce the misalignment between the driving AXMA and driven AXME axes, these driving AXMA and driven AXME axes advantageously approaching each other, possibly to the point of being coaxial and aligned.

[0085] Thus, when the torque transmitted by the transmission system 10 increases, the misalignment of the driving AXMA and driven AXME axes is reduced, which has the effect of limiting the appearance of parasitic forces in the transmission system 10, and of limiting the effects of these parasitic forces on the transmission system 10 and its components, such as the guide devices 5-8, the casing 54, the drive device(s) 31, 32 and the teeth 14, 24, as well as the input 514 and output 524 toothed wheels, if any. As a result, these components are less stressed, which makes it possible to limit maintenance operations and consequently the cost of using the transmission system 10 according to the invention.

[0086] [Fig. 3] schematically represents the transmission system 10 when stopped and in operation. The driving shaft 1 and the driving toothed wheel 141 are shown in solid lines for the transmission system 10 when stopped, and with dotted lines for the transmission system 10 in operation. The driven shaft 2 and the connecting device 3 are unchanged whether the transmission system 10 is stopped or in operation.

[0087] When the transmission system 10 is stopped, the driving shaft 1, shown in a solid line, and the driven shaft 2 are not coaxial, the driving AXMA and driven AXME axes being parallel to each other.

[0088] When the transmission system 10 is operating, the forces experienced are such that the driving shaft 1, shown in dotted lines, and the driven shaft 2 become substantially coaxial and aligned, the driving AXMA and driven AXME axes also being substantially aligned.

[0089] To obtain such operation under loads of the transmission system 10, the misalignment between the driving AXMA and driven AXME axes was defined as a function of the relative displacements under loads of the driving AXMA and driven AXME axes during of the operation of this transmission system 10. These movements of the driving AXMA and driven AXME axes could be determined by simulations and / or tests.

[0090] A method of limiting parasitic forces in the transmission system can make it possible to determine the value of this initial misalignment.

[0091] This method firstly comprises a step of determining a relative displacement between the driving axis AXMA and the driven axis AXME during operation of the transmission system. The value of this relative displacement can be determined by simulation and / or testing.

[0092] This relative movement between the driving axis AXMA and the driven axis AXME can be determined, for example, when the transmission system 10 is in one or more particular operating phases of the transmission system 10. This or these particular operating phases can, for example, comprise the operating phase(s) that are the most penalizing in terms of forces undergone, and / or the most used.

[0093] According to a first example, this relative displacement can be, for example, determined when the transmission system 10 transmits a mechanical torque from the driving shaft 1 to the driven shaft 2 greater than or equal to a predetermined torque.

[0094] According to a second example, this relative displacement can be determined as being equal to an average relative displacement of the driving AXMA and driven AXME axes when the transmission system transmits a mechanical torque from the driving shaft to the driven shaft within a predetermined interval.

[0095] According to a third example, this relative displacement can be determined as being equal to an average relative displacement of the driving AXMA and driven AXME axes when the transmission system operates according to the operating phase(s) which are the most penalizing in terms of forces undergone or the most used.

[0096] Then, this method comprises a step of misaligning the driving axis AXMA and the driven axis AXME as a function of this relative displacement when the transmission system is stopped.

[0097] This method thus makes it possible to determine the value of the misalignment to be applied between the driving axis AXMA and the driven axis AXME at rest. For example, this misalignment is equal to the value of the relative displacement previously determined.

[0098] To obtain this misalignment of the driving AXMA and driven AXME axes, the first center of rotation of at least one of the first guide devices 5, 6 and / or the second center of rotation of at least one of the second guide devices 7, 8 is offset relative to a theoretical position in which the driving AXMA and driven AXME axes are coaxial and aligned. This offset of a first or second center of rotation can be achieved only radially relative to the driving AXMA axis or the driven AXME axis, namely in a plane passing through this theoretical position and per pendicular to the driving axis AXMA or to the driven axis AXME. This offset of the first or second center of rotation can also combine such a radial offset and an axial offset, parallel to the driving axis AXMA or to the driven axis AXME.

[0099] The center of rotation of only one of the first and second guide devices 5-8 can be offset to obtain the desired misalignment between the driving AXMA and driven AXME axes, in particular when the driving AXMA and driven AXME axes are not parallel to each other when the transmission system 10 is stopped.

[0100] The first centers of rotation of all the first guide devices 5, 6 or the second centers of rotation of all the second guide devices 7, 8 may be offset to obtain the desired distance between the driving AXMA and driven AXME axes. The offsets will be the same for each of the first guide devices 5, 6 or the second guide devices 7, 8 if the driving AXMA and driven AXME axes are parallel to each other when the transmission system 10 is stopped. The offsets will be different for each of the first guide devices 5, 6 or the second guide devices 7, 8 if the driving AXMA and driven AXME axes are not parallel to each other when the transmission system 10 is stopped.

[0101] Whether the driving AXMA and driven AXME axes are parallel or not, the first centers of rotation of all the first guide devices 5, 6 and the second centers of rotation of all the second guide devices 7, 8 can also be offset.

[0102] Several technical solutions are possible to obtain the desired offset of a center of rotation of one of the guide devices 5-8.

[0103] According to a first variant, the offset of the first or second center of rotation of one or more of the first guide devices 5, 6 and / or second guide devices 7, 8 can be obtained by virtue of the position of the first housings 15, 16 and / or the corresponding second housings 17, 18. In this case, the first housings 15, 16 and the second housings 17, 18 are not coaxial when the transmission system 10 is stopped, as shown in [Fig. 1]. The first housings 15, 16 are in this case centered on the driving axis AXMA and the second housings 17, 18 are centered on the driven axis AXME. This offset of the first or second center of rotation of one or more of the guide devices 5-8 is, for example, obtained by machining the corresponding housing(s) 15-18 in the casing 54.

[0104] According to a second variant, the offset of the first or second center of rotation of one or more of the first guide devices 5, 6 and / or second guide devices 7, 8 can be obtained by means of the guide device(s) 5-8 concerned. In this case, all of the first housings 15, 16 and the second housings 17, 18 are coaxial when the transmission system 10 is stopped. On the other hand, the first or second guide device(s) 5-8 concerned are eccentric, namely that their first or second center of rotation is eccentric relative to the center of the housing 15-18 in which this guide device 5-8 is positioned.

[0105] For example, [Fig.4] represents a first housing 15 and a first guide device 5 according to this second variant. The first housing 15 is coaxial with the other first and second housings 16-18 and centered for example on the driven axis AXME. The first guide device 5 comprises an eccentric outer ring 53 and a “conventional” inner ring 52, namely annular. The outer peripheral surface 531 of this eccentric outer ring 53 is centered on the driven axis AXME while the inner peripheral surface 532 of this eccentric outer ring 53 is centered on the driving axis AXMA. These outer peripheral surfaces 531 and inner 532 are then eccentric and therefore not coaxial. Conversely, the “conventional” inner ring 52 comprises inner peripheral surfaces 522 and outer 521 which are coaxial.The eccentricity of the eccentric outer ring 53 is equal to the desired offset of the first center of rotation of this first guide device 5.

[0106] According to a third variant, the offset of the first or second center of rotation of one or more of the first guide devices 5, 6 and / or second guide devices 7, 8 can be obtained using an eccentric ring 9 inserted between the housing 15-18 and the guide device 5-8 concerned. In this case, all the first housings 15, 16 and the second housings 17, 18 are coaxial when the transmission system 10 is stopped. The first housings 15, 16 and the second housings 17, 18, and on the other hand one of the first guide devices 5, 6 and the second guide devices 7, 8 are, according to this third variant, “conventional” namely that their inner rings 52, 62, 72, 82 and outer rings 53, 63, 73, 83 are respectively coaxial two by two.

[0107] The eccentric ring 9 comprises two external 91 and internal 92 circular peripheral surfaces which are eccentric with respect to each other, namely that their respective centers are not coaxial.

[0108] An example of a third variant is shown in [Fig. 5], in which such an eccentric ring 9 is positioned in a first housing 15 and around a first guide device 5. The first housing 15 is coaxial with the other first and second housings 16-18 and centered for example on the driven axis AXME. The external peripheral surface 91 of the eccentric ring 9 is centered on the driven axis AXME while the internal peripheral surface 92 of this eccentric ring 9 is centered on the driving axis AXMA. The external 91 and internal 92 peripheral surfaces are thus eccentric and therefore not coaxial. The eccentricity of the eccentric ring 9 is equal to the desired offset of the first center of rotation of this first guide device 5.

[0109] According to a fourth variant, the transmission system 10 may comprise a casing 54 provided with at least a first subassembly 57 and a second subassembly 58. The first subassembly 57 comprises said at least two first housings 15, 16 and the second subassembly 58 comprises said at least two second housings 17, 18.

[0110] The first subassembly and the second subassembly are fixed to each other so that the driving axis AXMA and the driven axis AXME are misaligned when the transmission system 10 is stopped. An example of a fourth variant is shown in [Fig.l].

[0111] The first subassembly 57 and the second subassembly 58 are thus offset relative to each other, relative to a position in which the driving axis AXMA and the driven axis AXME would be aligned when the transmission system 10 is stopped. For example, the first subassembly 57 and the second subassembly 58 are offset relative to each other, relative to a position in which the first housings 15, 16 and the second housings 17, 18 are coaxial when the transmission system 10 is stopped. Consequently, following this offset, the first housings 15, 16 and the second housings 17, 18 are not coaxial when the transmission system 10 is stopped.

[0112] The transmission system 10 may equip an aircraft 100, and in particular a power transmission box 50 of this aircraft 100, as shown in [Fig. 6]. The power transmission box 50 allows an engine 55 to drive a lift rotor 102 in rotation. The aircraft 100 may also include an auxiliary rotor 103 capable of being driven in rotation by the engine 55, via the power transmission box 50 for example. The power transmission box 50 includes an input shaft 51 mechanically connected to the engine 55 and an output shaft 61 mechanically connected to the lift rotor 102.

[0113] When the transmission system 10 equips such an aircraft 100, the particular operating phases of the transmission system 10, used in particular to determine the relative movement between the driving AXMA and driven AXME axes, can be associated with flight phases of the aircraft 100, in particular the most penalizing flight phases, which can be the takeoff, cruising flight, or even hovering flight phases, and the most used flight phase which is generally cruising flight.

[0114] Naturally, the present invention is subject to numerous variations as to its implementation. Although several embodiments have been described, it is understood that it is not conceivable to exhaustively identify all possible modes. It is of course conceivable to replace a means described by an equivalent means without departing from the scope of the present invention and the claims.

Claims

Claims

1. Transmission system (10) comprising a driving shaft (1) as well as a driven shaft (2) and a connecting device (3) mechanically connecting said driving shaft (1) and said driven shaft (2), said connecting device (3) allowing said driving shaft (1) to rotate said driven shaft (2), said transmission system (10) comprising at least two first housings (15, 16) and at least two first guide devices (5, 6) guiding said driving shaft (1) in rotation around a driving axis (AXMA), each first housing (15, 16) receiving one of said first guide devices (5, 6), said transmission system (10) comprising at least two second housings (17, 18) and at least two second guide devices (7, 8) guiding said driven shaft (2) in rotation around a driven axis (AXME), each second housing (17, 18) receiving one of said second guide devices (7, 8),characterized in that said driving shaft (AXMA) and said driven shaft (AXME) are misaligned when said transmission system (10) is stopped, said driving shaft (1) as well as said driven shaft (2) and said connecting device (3) being located around a center line (LM) of said driving shaft (AXMA) and said driven shaft (AXME).,

2. Transmission system (10) according to claim 1, wherein said connecting device (3) comprises at least one driving device (31,32) allowing said driving shaft (1) to transmit a mechanical torque towards said driven shaft (2).

3. A transmission system (10) according to claim 2, wherein said connecting device (3) comprises an intermediate shaft (35), said at least one driving device (31,32) comprising a first driving device (31) and a second driving device (32), said first driving device (31) transmitting a mechanical torque from said driving shaft (1) to said intermediate shaft (35) and said second driving device (31,32) transmitting a mechanical torque from said intermediate shaft (35) to said driven shaft (2).

4. Transmission system (10) according to any one of claims 2 to 3, wherein said at least one drive device (31,32) comprises a device chosen from a list comprising splines, at least one key, one bolted assembly, one freewheel, one clutch.

5. A transmission system (10) according to any one of claims 1 to 4, wherein said driving axis (AXMA) and said driven axis (AXME) are parallel to each other when said transmission system (10) is stopped.

6. A transmission system (10) according to any one of claims 1 to 4, wherein said driving axis (AXMA) and said driven axis (AXME) are inclined relative to each other when said transmission system (10) is stationary.

7. A transmission system (10) according to any one of claims 1 to 6, wherein each first guide device (5,6) has a first center of rotation about which a first inner ring (52,62) of said first guide device (5,6) rotates relative to a first outer ring (53,63) of said first guide device (5,6), each second guide device (7,8) has a second center of rotation about which a second inner ring (72,82) of said second guide device (7,8) rotates relative to a second outer ring (73,83) of said second guide device (7,8), said driving axis (AXMA) passing through said first centers of rotation, said driven axis (AXME) passing through said second centers of rotation.

8. A transmission system (10) according to any one of claims 1 to 7, wherein said first housings (15,16) and said second housings (17,18) are cylindrical and are coaxial when said transmission system (10) is stopped, and at least one of said first guide devices (5,6) and said second guide devices (7,8) comprises an eccentric outer ring (53,63,73,83), said eccentric outer ring (53,63,73,83) comprising an outer peripheral surface and an inner peripheral surface which are eccentric relative to each other.

9. A transmission system (10) according to any one of claims 1 to 7, wherein said first housings (15,16) and said second housings (17,18) are cylindrical and are coaxial when said transmission system (10) is stopped, and said transmission system (10) comprises at least one eccentric ring (9) inserted between on the one hand one of said first housings (15,16) and said second housings (17,18), and on the other hand one of said first guide devices (5,6) and said second guide devices (7,8), said eccentric ring (9) comprising an external peripheral surface (91) and an internal peripheral surface (92) which are eccentric relative to each other.

10. A transmission system (10) according to any one of claims 1 to 7, wherein said first housings (15,16) and said second housings (17,18) are cylindrical, said first housings (15,16) are centered on said driving axis (AXMA) and said second housings (17,18) are centered on said driven axis (AXME) when said transmission system (10) is stopped.

11. A transmission system (10) according to any one of claims 1 to 7, wherein said transmission system (10) comprises a housing (54) composed of several assembled subassemblies (56-59) including at least a first subassembly (57) and a second subassembly (58), said first subassembly (57) comprising said at least two first housings (15, 16) and said second subassembly (58) comprising said at least two second housings (17, 18), and said first subassembly (57) and said second subassembly (58) are fixed to each other so that said driving shaft (AXMA) and said driven shaft (AXME) are misaligned when said transmission system (10) is stopped, said first subassembly and said second subassembly being offset relative to each other, relative to a position in which said driving shaft (AXMA) and said driven axis (AXME) would be aligned when said transmission system (10) is stopped.

12. A power transmission box (50) comprising a transmission system (10) according to any one of claims 1 to 11.

13. Aircraft (100) comprising a mechanical power transmission system (10) according to any one of claims 1 to 11.

14. Method for limiting parasitic forces in a transmission system (10) according to one of claims 1 to 11, said method comprising the following steps: - determination of a relative displacement between said driving axis (AXMA) and said driven axis (AXME) during operation of said mechanical system (10), - misalignment, when said transmission system is stopped, of said driving axis (AXMA) and said driven axis (AXME) as a function of said relative displacement.

15. A method according to claim 14, wherein, in said determining a relative displacement between said driving shaft (AXMA) and said driven shaft (AXME), said relative displacement between said driving shaft (AXMA) and said driven shaft (AXME) is determined when said transmission system (10) transmits a mechanical torque from said driving shaft (1) to said driven shaft (2) greater than or equal to a predetermined torque.

16. A method according to claim 14, wherein, in said determining a relative displacement between said driving axis (AXMA) and said driven axis (AXME), said relative displacement between said driving axis (AXMA) and said driven axis (AXME) is equal to an average relative displacement of said driving (AXMA) and driven (AXME) axes when said transmission system (10) transmits a mechanical torque from said driving shaft (1) to said driven shaft (2) within a predetermined interval.

17. A method according to claim 14, wherein, upon said determination, said relative displacement between said driving axis (AXMA) and said driven axis (AXME) is equal to an average relative displacement of said driving (AXMA) and driven (AXME) axes when said transmission system (10) is in one or more predetermined operating phases. Mechanical power transmission system, power transmission box and aircraft

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