A cleaning system for a combine harvester. The cleaning system includes a sieve assembly, a sieve drive, and a louver drive. The sieve assembly includes at least one sieve for receiving a flow of harvested and threshed crop. The sieve includes a plurality of spaced apart louvers forming apertures for separating grain from the flow of the harvested crop, the louvers extending in a direction substantially perpendicular to the flow of the harvested crop. The sieve drive is arranged to apply a reciprocating back and forth sieve movement, at a first frequency f1, to the sieve for propagating the flow of crop from a front end of the sieve to a back end of the sieve. The louver drive is configured to apply a reciprocating louver movement to the louvers at a second frequency f2, whereby f2>f.
