Gearbox device for agricultural machines and related method

A method for changing the transmission ratio of a gearbox device for an agricultural machine, the gearbox comprising a primary shaft, a secondary shaft, at least one intermediate shaft, and a motion transmission element moveable between at least two operating positions in which it sets up intermeshing drive lines between the primary shaft and the secondary shaft with different gear ratios, at least one of the drive lines incorporating the intermediate shaft, the motion transmission element passing through at least one neutral position during its movement between the at least two aforementioned operating positions;

• device wherein changing the transmission ratio of the gearbox device to a first ratio consists first in waiting for the primary, secondary, and intermediate shafts to stop rotating, then to move the motion transmission element axially in the direction of the secondary shaft to the neutral position, then to exert an axial force on the motion transmission element in the same direction, while manually operating the pivoting means making it possible to make at least one of the primary, secondary, and intermediate shafts pivot around its respective rotation axis, until the motion transmission element and the intermediate shaft are coupled.