Power transmission gearbox presenting a modifiable speed of rotation at its outlet, and a corresponding method of operation

1. A power transmission gearbox for rotorcraft for transmitting rotary movement from at least two engine members (MG, MD) to a main shaft ( 1 ) with determined reduction ratios, wherein the gearbox presents at least two transmission systems (G, D) for imparting rotary movement to the main shaft ( 1 ), each of the transmission systems comprising:

an inlet shaft ( 3 ) designed to be driven in rotation by an engine member (MG, MD);

at least two reduction stages ( 4 , 5 ) presenting different reduction ratios (k 2 , k 1 );

a first gearwheel ( 6 ) connected to the inlet shaft ( 3 ) via a declutchable freewheel ( 7 ), said gearwheel ( 6 ) meshing with a first toothed wheel ( 8 ) secured to an intermediate shaft ( 9 ), thereby defining at least a first reduction stage ( 4 );

a second gearwheel ( 10 ) connected to the inlet shaft ( 3 ) via a simple freewheel ( 11 ), said second gearwheel ( 10 ) meshing with a second toothed wheel ( 12 ) secured to the intermediate shaft ( 9 ), thereby defining the second reduction stage ( 5 );

a main freewheel ( 13 ) mounted on the intermediate shaft ( 9 ) to rotate a complementary toothed wheel ( 14 ) secured to the main shaft ( 1 ); and

a clutch mechanism for the declutchable freewheel ( 7 ) for the purpose, depending on circumstances, of putting the inlet shaft ( 3 ) into engagement with said declutchable freewheel ( 7 ) or for drivingly isolating the inlet shaft ( 3 ) from said declutchable freewheel ( 7 ).

2. A power transmission gearbox according to claim 1 , wherein the clutch mechanism is mounted on the inlet shaft ( 3 ).

3. A power transmission gearbox according to claim 1 , wherein the clutch mechanism is actuated via a hydraulic actuator ( 15 ).

4. A power transmission gearbox according to claim 3 , wherein the clutch mechanism comprises a mechanism for engaging and disengaging the inlet shaft ( 3 ) with an outlet shaft that is coaxial with the inlet shaft over at least a portion of its length.

5. A power transmission gearbox according to claim 1 , wherein the main shaft ( 1 ) is designed to drive a rotary wing or at least one rotor, e.g. carrying blades ( 2 ).

6. A power transmission gearbox according to claim 1 , wherein each transmission system has n reduction stages, one simple freewheel ( 11 ), one main freewheel ( 13 ), and n−1 declutchable freewheels ( 7 ), where n is the number of available speeds, i.e. an integer greater than or equal to 2.

7. A control method for modifying the speed of rotation of a main shaft of a power transmission gearbox in accordance with claim 1 , wherein the method consists in:

j1) reducing the speed of rotation of the left engine member (MG) to a value Ne 1 , starting from a value Ne, while conserving the value Ne for the speed of rotation of the right engine member (MD);

j2) declutching the declutchable freewheel ( 7 ) of the left transmission system (G);

j3) increasing the speed of rotation of the left engine member (MG) to the value Ne and keeping said speed of rotation constant;

j4) decreasing the speed of rotation of the right engine member (MD) to a value less than Ne, e.g. equal to Ne 1 ;

j5) declutching the declutchable freewheel ( 7 ) of the right transmission system (D); and

j6) increasing the speed of rotation of the right engine member (MD) to the value Ne.

8. A control method according to claim 7 , consisting in implementing steps j1) to j6) or i1) to i5) with the help of a computer.

9. A control method for modifying the speed of rotation of a main shaft of a power transmission gearbox in accordance with claim 1 , wherein the method consists in:

i1) reducing the speed of rotation of the left engine member (MG) to a value Ne 1 , starting from a value Ne, while conserving the value Ne for the speed of rotation of the right engine member (MD);

i2) declutching the declutchable freewheel ( 7 ) of the left transmission system (G);

i3) increasing the speed of rotation of the left engine member (MG) to a value Ne 2 , greater than the value Ne;

i4) declutching the declutchable freewheel ( 7 ) of the right transmission system (D); and

i5) decreasing the speed of rotation of the left engine member (MG) to the value Ne.

10. A motor assembly for a rotorcraft of the helicopter kind, the assembly comprising a main transmission gearbox in accordance with claim 1 and a respective engine member (MG, MD) of the turbine engine or piston engine kind associated with each of the transmission systems.

11. A helicopter or a rotorcraft including an engine assembly according to claim 10 .

12. A helicopter or a rotorcraft including a power transmission gearbox in accordance with claim 1 .