The device has an electric cable (10) whose one portion is provided corresponding to a metal sleeve (3) such that the sleeve abuts against flanges (4) by longitudinal traction of the cable. The flanges are fixedly connected together by the two non-deformable rods (7), where one of the rods is located above the cable and the other rod is located below the cable. The sleeve is engaged between the flanges in another sleeve. A spring unit is engaged in the sleeve to provide a corresponding portion of curved convexity directed toward an opening.
