A marking device (10) comprises a frame (1) (of frame plates (12, 14)) to which are fixed first and second motors (16, 18). A carriage frame (40) is pivoted in the frame about a screw axis (22). A marking head (30) is mounted on the carriage frame for movement in a direction parallel the screw axis. A drive screw (20) is rotationally mounted in the frame along the screw axis and is driven by the first motor (16). The carriage frame is journalled on said drive screw to permit said pivoting about the screw axis. The marking head is driven by rotation of the screw, the carriage frame being pivoted by the second motor. The carriage frame (20) comprises a bar (32) on which the marking head slides. The bar is disposed parallel the drive screw. The bar has at each end an arm (34, 36) pivoted on bearings (35, 37) on the drive screw. One arm (36) has an extension (66) fixed to a belt (58) disposed around two idler pulleys (62, 64) and a motor pulley (60) driven by the second motor. The extension has a contact surface (68) having a curvature centred on the drive screw and having both radius and circumferential extent that is sufficient to approach contact with the belt around the idler pulleys. By this means, the tension of the belt is maintained throughout its pivoting of the arm extension (66).