Abstract: A linear unit (1) comprising a profiled rail (2) along which a carriage (3) is arranged for sliding motion backwards and forwards by means of a reversible screw (4) which actuates the carriage and on either side of the carriage passes through a bore (7) in a screw support (8). A pull rod (24) displaces the screw support backwards and forwards along the profiled rail between to end stop positions, and the screw support serves as a support bearing carrying the screw between bearings (10) supporting the screw at its ends. A self-centering device for automatic centering of the screw in the screw support is formed with a self-centering element (11) mounted in the screw-support bore, said element slidingly abutting against the screw both when the screw is rotating in either one of its directions of rotation and when the screw support assumes its end stop positions.