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In a main transmission gearbox for a helicopter having two engines, a lifting rotor and an anti-torque rotor, each of the two engines drives a main kinematic chain comprising from upstream down, an input shaft, a twistable shaft, a gear train forming a first reduction stage having a first bevel pini

In a main transmission gearbox for a helicopter having two engines, a lifting rotor and an anti-torque rotor, each of the two engines drives a main kinematic chain comprising from upstream down, an input shaft, a twistable shaft, a gear train forming a first reduction stage having a first bevel pinion meshing with a second bevel pinion which drives in rotation a vertical shaft bearing having a third pinion in mesh with a gearwheel which is common to the two main chains and forms with the third pinions a second reduction stage. The torque received at the gearwheel is transmitted to the driving shaft of the main rotor by way of a supplementary reduction stage of epicyclic type. A third chain comprising two input pinions each of which is mounted upon one of the input shafts and an intermediate pinion which drives the driving shaft of the anti-torque rotor, ensures the distribution of power over the two main chains in the event of breakdown of one engine.

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A main transmission gearbox for a helicopter having a lifting rotor, an anti-torque rotor and two engines each of which has an output shaft, said gearbox comprising: a vertical driving shaft for coupling to said lifting rotor, two main kinematic chains for coupling the respective engines to the vert

A main transmission gearbox for a helicopter having a lifting rotor, an anti-torque rotor and two engines each of which has an output shaft, said gearbox comprising: a vertical driving shaft for coupling to said lifting rotor, two main kinematic chains for coupling the respective engines to the vertical driving shaft, each main kinematic chain comprising an input stage having an input shaft arranged to be driven in rotation by the output shaft of the associated engine, a first reduction stage having a first transmission shaft driven in rotation by said input shaft, a first bevel pinion driven by said first transmission shaft, a second bevel pinion meshing with said first bevel pinion and a second transmission shaft driven by said second bevel pinion, and a second reduction stage including a third pinion driven by said second transmission shaft and a gearwheel which meshes with said third pinions of both of said main kinematic chains and which is coupled to said vertical driving shaft and a third kinematic chain comprising a train of gears further coupling the two main kinematic chains together, said train of gears comprising an input pinion mounted on and for rotation with said input shaft of each main kinematic chain and at least one intermediate pinion disposed between and in geared connection with said input pinions to couple said input pinions together, each of said first transmission shafts extending between said first bevel pinioin and said input pinion and being flexible in a torsion, said at least one intermediate pinion directly driving a drive shaft of said anti-torque rotor.