Quality requirements to be met by roadbuilding, and concrete aggregates, in so far the maximum admissible content of the contaminants is concerned, necessitate installation of effective screen-separators. Their efficacy depends on proper choice of oscillating system and of screen-design. Primary separators are not only used for removing dirt but are also meant for relieving primary and secondary crushing equipment. THe paper describes a number of plants which feature high feed rates and -sizes.