A hub (16) can be fixed to a blade carrier disc (11) and is mounted on a shaft (10) rotationally operated about a vertical axis. To transfer rotary movement from the shaft (10) to the hub (16) a plate (17) rotatable with the shaft and slidable along it is provided adjacent to the hub (16). A spring (19) pushes the plate towards the hub (16). The plate (17) and the hub (16) have corresponding interface surfaces which are inclined with respect to the axis of rotation or curved to convert movement of relative rotation between the hub (16) and the plate (17) into a relative separating movement in an axial direction. In a normal operating condition, the first and second interface surfaces are axially close and mutually engaged to transfer the rotary movement from the shaft (10) to the hub (16). Following an impact of the blades of the disc (11) with an external body, the first interface surfaces are disengaged from the second surfaces, axially spaced apart and angularly offset with respect thereto, so that the blade carrier disc and the hub are rotationally disengaged from the plate and the shaft.
