The device has a shaft (1) i.e. hollow tube, carrying a flail (2) composed of a mounting unit i.e. non-working portion (23), with a mounting orifice (21). An axial support (11) i.e. semi-octagonal rod, and the orifice have non-circular portions (12, 22) that are fitted in one another under the action of centrifugal force, for maintaining the unit in work position. A section of the orifice is larger than a section of the support to authorize the displacement of the unit around the support when the unit is brought to another position, by the presence of an obstacle on the course of the flail.
