A screw feeder with a reversibly rotating axle (14) for feeding material in a definite direction. The axle (14) is provided with feed elements (21,25) having two feeding flanks, each formed as a portion of a right-hand and, respectively, left-hand thread screw. The feed elements (21,25) preferably are attached on annular members (20,24), which are arranged one after the other along the axle (14). Each feed element (21,25) is rotatable about the axle (14) through a definite angle in relation to adjacent feed elements, so that the feed direction is maintained irrespective of the rotation direction of the axle (14). At least one of the feed elements being operably coupled to the axle for rotation therewith.