Device for changing the transmission ratio between a turbine shaft and a shaft of a starter-generator of an aeroengine

1. A device for changing the transmission ratio between a turbine shaft and a shaft of a starter-generator of an aeroengine, the device comprising:

first and second fixed gear wheels carried by the shaft of the starter-generator;

first and second idler gearwheels carried by the turbine shaft, and meshing respectively with the first and second fixed gearwheels to define different gear ratios;

a changeover sleeve interposed between the idler gearwheels and mechanically coupled to the turbine shaft, the changeover sleeve being movable in translation on the turbine shaft between two coupling positions of: a position in which it meshes with the first idler gearwheel, and an opposite position in which it meshes with the second idler gearwheel; and

means for causing the changeover sleeve to move in translation in automatic manner from either one of its coupling positions to the other one when the sum of torques between the turbine shaft and the starter-generator shaft changes sign.

2. A device according to claim 1 , wherein each of the side faces of the changeover sleeve carries dogs, each dog comprising:

a first plane face for engaging against a plane torque-transmission face of a tooth carried by one of the idler gearwheels when the sleeve is in one of its coupling positions; and

a second plane face opposite from the first face and configured, when the sum of the torques between the turbine shaft and the shaft of the starter-generator changes sign, to slide over a plane decoupling face of a tooth of one of the idler gearwheels opposite from its torque-transmission face.

3. A device according to claim 2 , wherein the second faces of the dogs of the changeover sleeve are substantially parallel to the axis of rotation of the changeover sleeve, while the decoupling faces of the teeth of the idler gearwheels are inclined relative to the axis of rotation in the direction of rotation of the sleeve.

4. A device according to claim 3 , wherein the decoupling faces of the teeth of the idler gearwheels form an angle with the axis of rotation of the changeover sleeve, which angle lies in a range of 45° to 60°.

5. A device according to claim 2 , wherein the decoupling faces of the teeth of the idler gearwheels are substantially parallel to the axis of rotation of the changeover sleeve, while the second faces of the dogs of the changeover sleeve are inclined relative to the axis of rotation in the direction of rotation of the sleeve.

6. A device according to claim 2 , wherein the first faces of the dogs of the changeover sleeve and the torque-transmission faces of the teeth of the idler gearwheels are substantially inclined relative to the axis of rotation of the changeover sleeve in the direction of rotation of the sleeve.

7. A device according to claim 6 , wherein the first faces of the dogs of the changeover sleeve and the torque-transmission faces of the teeth of the idler gearwheels form an angle with the axis of rotation of the changeover sleeve, which angle is about 5°.

8. A device according to claim 2 , further comprising means for stabilizing the changeover sleeve in its coupling positions.

9. An aeroengine comprising:

a turbine shaft;

a starter-generator; and

a device for changing transmission ratio between the turbine shaft and a shaft of the starter-generator according to claim 1 .