Gearbox

Gearbox comprising:

• a primary shaft (8) intended to be coupled to a power supply (3, 6);
• a secondary shaft (9) intended to be coupled to a power outlet (4, 5);
• a plurality of sets of gearwheels mounted on said primary (8) and secondary (9) shafts, said sets of gearwheels being able to be selectively meshed to establish a plurality of transmission ratios between said supply (3, 6) and said outlet (4, 5);
• said sets of gearwheels comprising a plurality of switching gearwheels (26, 27, 28) for said ratios, these gearwheels being mounted so as to move axially on said shafts (8, 9) to enable said ratios to be established;
• selection means (10 to 13;
• 29, 30, 31;
• 35, 36, 37;
• 42, 43, 44) coupled to said switching gearwheels (26, 27, 28) to cause them to make the appropriate axial movements according to the desired transmission ratio;
• said selection means (10, 12, 13) comprising a ratio selector shaft (10) rotationally mounted so as to be able to have at least as many selection positions as there are possible transmission ratios, said selector shaft (10) being therefore provided with cam forming means (35, 36, 37) coupled to said switching gearwheels (26, 27, 28) through the intermediary of counter-cam forming means (42, 43, 44);
• said gearbox being characterised in that it also comprises
• decoupling means (11, 12, 13;
• 57, 57, 58) for selectively disabling the coupling between each cam forming means (35, 36, 37) and each counter-cam forming means (42, 43, 44) associated with it, in order to enable the free rotation of said selector shaft (10) towards any ratio selection position, whatever the previously selected transmission ratio.