A safety device for farm machines provided with an apparatus (3) for treating materials of the type of agricultural products, equipped with at least one rotating working shaft (4) which faces the ground and is proximate thereto and is provided with means (5) for treating the materials. The device (1) comprises at least one movable element for actuating the interruption of the transmission of power from a drive unit of the working shaft (4) to the working shaft (4) itself; the actuation element is movable from a passive configuration, for free power transmission, to an active configuration, for interruption of power transmission, and vice versa. The safety device comprises an automatic apparatus for the timed actuation of the transition of the actuation element from the passive configuration to the active configuration. The automatic apparatus comprises at least first means for detecting the rotation rate of the working shaft (4) and second means for detecting the rotation rate of a driving shaft, comprised in the drive unit. The detection means are controlled by an electronic unit for the control and actuation of a mechanism (21) for the movement of the actuation element and the electronic unit is provided with a module for comparing the rotation rates for the timed sending, after a preset delay time monitored by a respective timer, of the command for transition of the movable element from the passive configuration to the active configuration, following the detection of a variation between the rates that exceeds a preset reference parameter, which represents a jamming of the working shaft (4). The delay time is shorter than the time estimated for manual intervention by an operator to remove the jam.
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