Abstract: A mechanically raked screen system is used for preliminary waste water treatment. A screen is formed of a multiplicity of mutually parallel screen bars. The screen bars are oriented substantially transversely relative to a flow direction of a flowing liquid and forming screen spaces of a given width therebetween. A rake assembly rakes the screenings from the screen. The rake assembly includes two endless chains which traveling about sprocket wheels disposed laterally of the screen, and a rake attached to and extending between the chains. The rake is oriented substantially perpendicularly to the screen and travels with the chains for repeatedly raking the screen. The tynes of the rake are integrally formed from a single metal piece together with the rake plate. The vertical screen bars have a trapezoidal cross section, such that the leading side facing against the flow of the waste water is about twice as wide as the trailing side.