A pedal articulated screen shaker assembly supporting a classifying screen at its top in an alignment over a collection receptacle is provided with an annular nested dust shielding combination suspended below the screen. The dust shielding combination comprises a conforming set of frustoconical elastomeric membrane segments in a nested arrangement with the exterior one of the segments perforated throughout its full circumference by perforations that are bridged by flexible flaps formed in the overlying interior segment that are lifted from their bridging alignment by the lateral movement imparted to the shielding combination.