An automatically operated braking device (F) for motion transmission means, in particular transmission means (A1,A2) operated by motor means of said tractors, includes mechanical working means (10), coupled with and sliding axially along a driving shaft (A1), and supporting, at one side, first means (11) for mutual coupling with meshing means (R1,R2), which are aimed at coupling the driving shaft (A1) with at least one driven shaft (A2), and supporting, at the other side, second means (12) for abutment against braking means (13), which are connected to the driving shaft (A1) near the mechanical working means (10) and the part opposite with respect to the meshing means (R1,R2). The mechanical working means (10) are aimed, when driving shaft (A1) is operated by the motor means to rotate ( omega 1), at defining, by the first means (11), mutual coupling and connection between the driving shaft (A1) and the meshing means (R1,R2), in order to operate the driven shaft (A2) and the working means connected therewith to rotate ( omega 2), and simultaneously, at ensuring disengagement of the second abutment means (12) from the braking means (13). The mechanical working means (10) are also aimed, when the motor means are disconnected or deactivated, at defining a partial and temporary disengagement of the first means (11) from the meshing means (R1,R2) due to slowing down of the driving shaft (A1) with respect to the meshing means (R1,R2), and at translating axially (N) toward the braking means (13), so as to define a friction engagement the latter and the second abutment means (12) to brake and stop the driving shaft (A1) and consequently, the meshing means (R1,R2) and then the driven shaft (A2) and relative working means connected therewith. <IMAGE>
