in the knot bearing pulp slurry due to the viscous drag exerted by the rotating screen 30 knot transporting flight 50 rotor shaft 14 and flight support bracket arms 16 however the induced rotary velocity of the knot containing pulp slurry which will increase in magnitude with distance traveled up screen 30 must be relatively lower than that of the inside surface of screen 30 this difference in velocity causes knots to be tangentially 34 dragged 34 over perforations 25 in screen 30 on to surface 27 and into zone 55 the relatively lower velocity of the 34 dragged 34 knots with respect to the upper transporting surface 51 of flight 50 results in the upward transport of the knots by the spiral transporting upper surface 51 of flight 50 through the unimpeded free flow channel zone 55 in screening chamber 20 and into stationary housing extension 40 fig4 and 6 illustrate the feature of the invention designed to eliminate the trapping and holding effect of the hydrodynamic screening forces which would otherwise result in knot transport interruptions in the screening chamber 20 fig4 shows a fragmentary schematic representation of the screen basket 30 its pattern of perforations 25 and surface 27 being one side of a zone 55 of reduced hydrodynamic screening forces this is described as a locus defining a knot collecting and transporting zone and because it indicates physical occlusion of certain of the perforations 25 of screen 30 in a pattern that conforms to the.