Planetary reducing gearset comprising a prestressed flexible support

A reducing assembly includes a planet carrier that is flexible and fixed, borne by a casing which surrounds it by two distinct connections with this casing, with a device for applying a torsional prestress to the planet carrier between these two connections. Prestressing the flexible planet carrier makes it possible to limit its level of mechanical stress when it is in service, in order to simplify its design and dimensioning.

TECHNICAL FIELD

The invention relates to a planetary reducing gearset with a flexible planet carrier for use particularly in a turbomachine such as a turbojet.

STATE OF PRIOR ART

In such an engine1shown inFIG.1, air is drawn in through an intake duct2to pass through a fan3comprising a series of rotating airfoils before being split into a central core flow and a bypass flow surrounding the core flow.

The core flow is then compressed in compression stages4and6before entering a combustion chamber7, after which it is expanded through a high-pressure turbine8and a low-pressure turbine9before being evacuated in the aft direction. The bypass flow is propelled directly in the aft direction by the fan in a flow stream delimited by the case11.

Such an engine is usually of the twin spool type: it comprises a so-called low-pressure rotor through which the fan3is coupled to the low-pressure turbine, surrounded by a so-called high-pressure rotor through which the compressor is coupled to the high-pressure turbine, these two rotors being independent of each other in rotation.

In some architectures, a reducing gearset is interposed between the low-pressure rotor and the fan, so that the blower rotates more slowly than the low-pressure turbine that drives it, thereby increasing the efficiency.

The reducing gearset that is of the planetary type, is located in the vicinity of the fan as shown schematically inFIG.2in which it is marked with the reference12. It comprises a sun gear13fixed to the low-pressure rotor, surrounded by planet gears14themselves surrounded by a ring gear16in the form of a ring with internal teeth fixed to the fan3. Each planet gear14is engaged with the sun gear13and with the ring gear16.

The planet gears14are carried by a planet carrier17that is attached to a fixed structural component18of the engine. This planet carrier17has an annular body19carrying the planet gears14, and a support21of revolution through which this body19is fixed to the structure18.

During operation, the sun gear13carried by the low-pressure rotor drives the planet gears14that rotate around fixed pins carried by the planet carrier17, that drive the fan3in rotation through the ring gear16with which they are engaged. This causes the fan3to rotate in the opposite direction to the low-pressure rotor and at a lower speed.

For reasons of overall dynamics, the support21is flexible along the radial direction to compensate for a lack of coaxiality of the fan and the low-pressure rotor, in practice this flexibility corresponds to a flexibility of the support21around the radial directions relative to the AX axis. As can be seen inFIG.21, the support21has a general bellows shape of revolution, in which the corrugations are crenelated when viewed in section.

The body19is provided with external teeth engaged in internal teeth of a ring gear22surrounding this body19and that is rigidly attached to the structure18. The engagement of these teeth that constitute a dog or spline type of coupling is provided with a significant circumferential clearance.

Under normal operating conditions, the torque applied on the body19results in a small torsion of the support21around the AX axis, so that the external teeth of the body19do not bear circumferentially on the teeth of the ring gear22, due to the significant circumferential clearance of these teeth. The torque applied on the body19is then entirely transferred to the structure18through the support21.

When the torque applied on the body19increases and exceeds normal conditions, the torsional deformation of the support21about the AX axis increases until the teeth on the body19bear circumferentially on the teeth of the ring gear22. Under these conditions, part of the torque is transmitted through the flexible support21and the remaining part of the torque is transmitted through the teeth, which avoids applying excessive stress on this flexible support.

In general, the fact that a high torque is applied to the satellite carrier although the satellite carrier has to have significant radial flexibility makes its sizing problematic.

The purpose of the invention is to provide a solution that simplifies the design of such a reducing gearset.

PRESENTATION OF THE INVENTION

To achieve this, the subject of the invention is a reducing gearset comprising a mobile sun gear and a mobile ring gear and a flexible and fixed planet carrier carrying planet gears that are engaged with the sun gear that they surround and with the ring gear that surrounds them, this planet carrier being connected to a case that surrounds it by at least two separate connections, with means to pretension the planet carrier between these two connections in torsion.

Thus, the invention can reduce the maximum torsional stress in the planet carrier during operation, a torsion being applied to the planet carrier when it is in service in the direction opposite to its prestress.

The invention also relates to a reducing gearset thus defined, wherein one of the two connections between the planet carrier and the case is a dog-type connection, comprising external teeth carried by the planet carrier and internal teeth carried by the case, and in which the prestress means are carried by teeth.

The invention also relates to a reduction gearset thus defined, wherein the prestress means are coil springs, each spring being circumferentially oriented and carried by a tooth, and interposed between an external tooth and an internal tooth that is consecutive to it.

The invention also relates to a reducing gearset thus defined, wherein each spring is carried by a tooth provided with a bearing face through which it bears on the tooth that is consecutive to it if the spring is fully compressed.

The invention also relates to a reducing gearset thus defined, wherein each spring surrounds a pin projecting from a tooth, this pin terminating at a bearing face.

The invention also relates to a reducing gearset thus defined, wherein each spring is engaged in a hole formed in the tooth that carries it.

The invention also relates to a reducing gearset thus defined, wherein the other connection between the planet carrier and the case is also a dog-type connection.

The invention also relates to a reducing gearset thus defined, comprising teeth provided with several springs.

The invention also relates to a reducing gearset thus defined, comprising at least one tooth provided with two springs separated radially or longitudinally on one face of this tooth.

Turbojet comprising a reducing gearset thus defined.

DETAILED DESCRIPTION

The reducing gearset according to the invention, marked with reference26inFIG.3, comprises a planet carrier27comprising firstly an annular body28carrying five planet gears29, and secondly a support31of revolution through which this body28is attached to a fixed component of the engine case32forming part of the turbomachine structure. The support31projects from a downstream face of the body, the upstream AM and downstream AV faces being defined relative to the direction of air flow in the engine, the planet carrier27possibly being built by assembling the body28with the support31through a flange.

This planet carrier27that is fixed in rotation around the longitudinal axis AX of the engine, surrounds a sun gear not shown and is part of a low pressure rotor that is not visible, and is surrounded by a ring gear not shown fixed to a fan of the engine. Each planet gear29is engaged with the sun gear and the ring gear not shown, the interior and the exterior being defined relative to the longitudinal axis AX, with the sun gear having a diameter less than the ring gear and consequently being closer to the AX axis.

As in the case inFIG.2, the low-pressure rotor thus rotates the fan through the planet gears29that rotate around fixed axes, the fan thus rotating more slowly than the low-pressure rotor and in the opposite direction.

The support31is sized to have a certain flexibility in order to compensate for the lack of coaxiality between the body28and the sun and ring gears. Therefore it also has some flexibility in torsion around the AX axis. As seen inFIGS.3to6, this carrier31is in the general shape of a bellows, with rectangular type corrugations when viewed in section.

The planet carrier assembly is locked in rotation by means of a dog type upstream connection L1located at the body28, and a downstream connection L2, also of the dog type located downstream from the support31. These two dog connections are sized with a significant circumferential clearance so that they can cooperate in distributing transmission of the torque applied by the planet carrier on the case32that surrounds it.

The body28is provided with upstream external teeth33that are engaged between upstream internal teeth34of the case32, the engagement of the upstream teeth33with the upstream teeth34constitutes a dog-type connection, but a significant clearance is provided in the circumferential and radial directions when stopped and during normal operation.

As can be seen in more detail in the example inFIG.4, each upstream external tooth33has a relatively simple general shape similar to a parallelogram, delimited by an upstream face36, a downstream face, a radially external face37, a lateral bearing face38and a secondary lateral face39. The upstream and downstream faces are normal to the AX axis, while the bearing face and the secondary face extend along planes passing through or at a small distance from the AX axis.

Similarly, in the example in the figures, each upstream internal tooth34is also parallelepiped in shape bounded by an upstream face41, a downstream face, a radially internal face, a lateral bearing face42and a secondary lateral face43.

Each upstream external tooth33carries a helical spring44that projects from its bearing face38being oriented towards and bearing on the bearing face42of the internal upstream tooth34that is interposed between these two consecutive upstream external teeth (33).

The downstream end of the support31is provided with downstream external teeth46that are engaged between downstream internal teeth47of the case32, forming the downstream dog connection T2that also has a significant circumferential clearance.

In the example in the figures, the shape of each downstream external tooth46is similar to a parallelogram, delimited by an upstream face, a downstream face, a radially external face, a lateral bearing face48and a secondary lateral face49, having the same orientations as the faces of the upstream external teeth. In the example in the figures, each downstream internal tooth47is also in the form of a parallelepiped delimited by an upstream face, a downstream face, a radially internal face, a lateral bearing face51and a secondary lateral face52having the same orientations as the faces of the upstream internal teeth.

The springs44press the lateral bearing faces38and42of the upstream external and internal teeth away from each other, tending to turn the planet carrier27relative to the case32in the anticlockwise direction in the figures, the ring gear that is connected in rotation to the engine fan rotating in the clockwise direction. Thus, in the rest state as inFIG.4, the springs44tend to rotate the planet carrier until the downstream internal and external teeth bear against each other at their secondary lateral faces49and52.

In this situation, the body28and the downstream end of the support31are considered as being at their respective reference positions around the AX axis. This reference position corresponds to a prestressed state of the support31, i.e. in which this support is deformed in torsion in the anticlockwise direction between connections L1and L2, this anticlockwise direction being opposite to its direction of deformation when it is in service.

When the assembly is assembled together, the springs44are compressed and a pretorsion is applied to the support31, this operation advantageously being carried out with special tools removed after assembly. The clearance at the downstream teeth is advantageously minimal, to facilitate assembly and limit the amplitude of the movement of the planet carrier.

When the engine is under normal operating conditions as shown inFIG.5, the body28is subjected to a nominal torque C resulting from forces applied on the teeth of the planet gears29, that tends to rotate it in the clockwise direction opposing the springs44. Under these conditions, the springs44are compressed until the downstream internal and external teeth46and47come into contact with each other through their bearing faces48and51respectively, and the springs44are compressed between the bearing faces38and42that they tend to force apart.

The assembly is sized so that, in this situation, the flexible support31undergoes a significant elastic torsional deformation about the AX axis between connections L1and L2, with its body28rotating by a small value in the clockwise direction relative to its downstream portion. Thus, part of the torque C is transmitted to the case32by the upstream teeth through the springs44, the complementary part of this torque being transmitted to the case by the downstream teeth bearing on each other.

In other words, when the torque C is equal to a nominal value, the compression of the springs44is too low to resist the entire torque: they are compressed until they bring the downstream internal and external teeth into contact.

Advantageously, this contact is made in the transient engine speed phase, so that at stable engine speed, there is no alternating contact and separation of the downstream teeth that could generate vibrations due to periodic torque variations.

The springs44are advantageously sized to transfer about half of torque C during normal operation, this size being dependent on the number and the stiffness of the springs.

When the torque C becomes greater than a predetermined nominal value, as inFIG.6, the springs44are fully compressed, and the support31undergoes maximum torsional deformation about the AX axis in the clockwise direction between its connections L1and L2. In the extreme situation shown inFIG.6, the springs44undergo maximum compression: each external tooth33comprises a pin53projecting perpendicular to its bearing face38surrounded by the spring44, and the end of which is a contact pad54bearing directly on the lateral face42of the next internal tooth34. This arrangement is shown in more detail inFIG.7, in which the axis is marked as reference53and its contact pad is marked as reference54.

In this situation, the support31undergoes an elastic deformation in torsion about the AX axis between connections L1and L2that is maximum for the size. The additional torque relative to the nominal torque is then transmitted to the case32by the upstream internal and external teeth, that then bear directly on each other.

In the example inFIG.7, the spring44surrounds a pin53projecting from the bearing face38, the end of which forms the bearing pad54of the external tooth33on the bearing face42of the next internal tooth34. Other arrangements are possible, as in the example inFIG.8, in which there is a blind hole56in the tooth33that passes through its bearing face38and in which the spring44is housed that projects significantly from this hole in the rest state. When the transmitted torque is greater than the nominal value, the tooth33then bears directly through its bearing face38on the bearing face42of the next internal tooth34, the spring44then being fully compressed in the blind hole56.

In the case inFIG.7, there is a clearance between the pin53and the spring44surrounding it, so that the spring can tolerate axial and radial movements of the body28relative to the case32fixed to the structure. In other words, the inside diameter of the spring44is larger than the outside diameter of the pin53. Similarly, in the case inFIG.8, a clearance is provided between the hole56and the spring44that fits in this hole: the inside diameter of the hole56is larger than the outside diameter of the spring44.

In both cases, the spring44is a coil spring that is fitted on the tooth that carries it, but it is also possible that each tooth can have two or three springs, in any appropriate arrangement, depending on the shape and extent of the bearing face38. For this purpose,FIG.9shows an arrangement wherein a tooth carries two springs44on its bearing face38that are radially separated from each other relative to the AX axis. In another configuration corresponding toFIG.10, the tooth carries two springs44arranged longitudinally side-by-side on its bearing face38, i.e. separated from each other along the AX axis.

In general, the first connection L1transfers torque beyond the nominal working torque, such torque also being called the ultimate torque. This first connection L1forms a limiter of the torsional deformation of the flexible support31, and consequently a limiter of the mechanical torsional stress applied to this support31.

In the examples in the figures, the springs44provide the torsional prestress of the planet carrier27, but other means may also be considered, for example such as a passive or active hydraulic or pneumatic system or a magnetic type system.