A method and an apparatus for separating a crop flow on at least one conveying and cleaning unit, particularly a top sieve, of a combine harvester, wherein the conveying and cleaning unit is excited to a longitudinal oscillation and a transverse oscillation. The transverse oscillation is controlled depending on at least one state, wherein least one state for controlling the transverse oscillation is the inclination of the combine harvester, wherein at least one further state for controlling the transverse oscillation is the grain purity, particularly the grain purity of a main crop flow. The transverse oscillation is pre-controlled depending on the inclination of the combine harvester and fine-tuned depending on the grain purity.