a drive assembly , especially for agricultural implements with rotatingly driven masses , has a shear pin coupling with a coupling hub and a coupling sleeve rotatable relative to the coupling hub . a ratchet gear supported on the coupling hub and rotatable relative to the coupling hub and the coupling sleeve . all three components are connected to one another by the head screw constituting the shear pin . a ratchet may engage a holding face of the ratchet gear . this results in the head screw being sheared . as a result , the coupling sleeve is disconnected from the coupling hub in respect of drive .