Coupling system between an operating machine and a planetary gearbox, comprising: - at least one axially rotatable machine shaft of an operating machine; - at least one slow gearbox shaft of a planetary gearbox connectable integrally in rotation with said machine shaft in an operational configuration; wherein said machine shaft and said slow gearbox shaft comprise at least one first coupling portion and at least one second coupling portion, respectively, substantially truncated cone-shaped and complementary to each other, said first coupling portion and said second coupling portion being conically coupled to each other in said operational configuration; - kinematic constraining means (18, 19) associated with said machine shaft and with said slow gearbox shaft, adapted to integrally constrain the latter to each other, wherein said constraining means (18, 19) comprise axial constraining means of the relative axial sliding of said machine shaft with respect to said slow gearbox shaft; characterized in that said constraining means of the relative sliding comprise: - at least one first ring nut associable with said machine shaft; - at least one second ring nut associable with said slow gearbox shaft; - fastening means coupleable in the operational configuration to said first ring nut and to said second ring nut to rigidly fix them to each other. - at least one axially rotatable machine shaft of an operating machine; - at least one slow gearbox shaft of a planetary gearbox connectable integrally in rotation with said machine shaft in an operational configuration; wherein said machine shaft and said slow gearbox shaft comprise at least one first coupling portion and at least one second coupling portion, respectively, substantially truncated cone-shaped and complementary to each other, said first coupling portion and said second coupling portion being conically coupled to each other in said operational configuration; - kinematic constraining means (18, 19) associated with said machine shaft and with said slow gearbox shaft, adapted to integrally constrain the latter to each other, wherein said constraining means (18, 19) comprise axial constraining means of the relative axial sliding of said machine shaft with respect to said slow gearbox shaft; - kinematic constraining means (18, 19) associated with said machine shaft and with said slow gearbox shaft, adapted to integrally constrain the latter to each other, wherein said constraining means (18, 19) comprise axial constraining means of the relative axial sliding of said machine shaft with respect to said slow gearbox shaft; characterized in that said constraining means of the relative sliding comprise: - at least one first ring nut associable with said machine shaft; - at least one second ring nut associable with said slow gearbox shaft; - fastening means coupleable in the operational configuration to said first ring nut and to said second ring nut to rigidly fix them to each other. - at least one first ring nut associable with said machine shaft; - at least one second ring nut associable with said slow gearbox shaft; - fastening means coupleable in the operational configuration to said first ring nut and to said second ring nut to rigidly fix them to each other.