Self-contained reduction gear with two ratios, and system with engine and reversible electrical machine including this reduction gear
The reduction gear includes a pair of gears installed in parallel between two shafts and each provided with a mobile wheel free to move axially to engage in turn with one of the shafts and to transmit movement with different ratios. Since the gear teeth are helical, opposite forces on the reduction gear make it changeover from one reduction ratio to the other. No external control is necessary. Application to mechanical transmissions between an engine and a reversible electrical machine (starter or generator), that starts the motor with a large speed reduction and generates electricity with a smaller reduction.
1. A system in an aircraft comprising:
a reversible electrical machine;
an engine; and
a mechanical transmission connecting two rotating shafts belonging to the reversible electrical machine and to the engine respectively, the reversible electrical machine being used both as a starter of the engine and a generator electrically loaded by the engine
wherein the mechanical transmission comprises a reduction gear mechanism comprising two gear sets with different tooth reduction ratios, each gear set having a first toothed wheel mounted on a first shaft of the two rotating shafts and a second toothed wheel mounted on a second shaft of the two rotating shafts, the first shaft being an output shaft of the engine and the second shaft being an output shaft of the reversible electrical machine,
the first toothed wheels of the two gear sets being mobile toothed wheels free to axially slide on the first shaft and both being provided with mesh relief present on a lateral side of the toothed wheels and capable of engaging with complementary relief on the first shaft at gear positions, the first toothed wheels of the two gear sets being installed free to rotate on the first shaft but fixed in rotation with the first shaft at said gear positions,
the first shaft carries a spacer extending between the first toothed wheels of the two gear sets and freely sliding around the first shaft, the spacer can come into contact with the two first toothed wheels simultaneously, keeping the two first toothed wheels at a distance that prevents the gear positions from being present simultaneously,
wherein teeth of the first toothed wheels are helical and have inclinations in a same tangential direction relative to the first shaft so that changeovers of the reversible electrical machine from a starter mode to a generator mode and from the generator mode to the starter mode are automatically accompanied with changeovers of a first toothed wheel, among said two first toothed wheels, that is in the gear position, without any external control of axial positions of the first toothed wheels along the first shaft.
2. The system according to claim 1 , wherein bearings support the first shaft, and the sliding movements of the first toothed wheels are limited by engagement positions on at a first side of the first shaft closer to the spacer and stops on the first shaft on a second side of the first shaft closer to one of the bearings.
3. The system according to claim 1 , wherein the complementary relief on the first shaft is tapered in a tangential direction of the first shaft at least at one end part.
4. The system according to claim 1 , wherein the complementary relief on the first shaft includes a groove having first and second lateral faces, a first end of the first lateral face and a second end of the second lateral face each having a taper with the same inclination, and a second end of the first lateral face and a first end of the second lateral face having a taper with the same inclination.