Abstract: An apparatus for separating particulate or lump material by size having a passage for the material, downwardly defined by a plurality of rotationally drivable discs which are arranged in several rows. The discs in one row mesh with clearance between the discs in the nearest adjacent rows on either side, such that together the discs form a screen extending along the passage from an inlet for the material to be screened to an outlet for lumps of the material which are too large to fall through the screen. The principle axes of the disc rows are oriented in the flow direction of the material through the passage and locate the discs in a flat-sided trough-like configuration. The discs are adapted to be driven in such a direction that they impart to the material on the screen a transverse arching movement upwards and towards both sides of the passage, while the material simultaneously moves towards said outlet.