such a screen include an outer diameter of 60 inches for the outer tubular wall 50 an axial length of 60 inches for the inner tubular wall 51 and a diameter for the tubular wall 51 of approximately 32 inches the hub 65 should be positioned to provide the open space 66 within wall 51 with an axial length of 22 inches and the cone 70 should be proportioned to provide the space 71 with maximum axial dimension of 22 inches in operation the feed stock enters the inlet chamber 40 tangentially through the inlet port 44 and swirls around that chamber before moving into the screening chamber 30 the combination of centifugal force and gravity will therefore cause any heavy reject particles which may still be in the stock to collect along the wall of that portion of housing 10 which forms the outside of the inlet chamber and then to exit through the reject outlet 45 the rotor assembly 33 will be driven to rotate in the same direction as the entering stock i e clockwise as viewed in fig2 and in the zone adjacent the inside surface of the screening cylinder the vanes 60 will operate in the desired manner to create alternating positive and negative pulsations adjacent the inner ends of the perforations in the cylinder 30 to minimize the possibility of blocking of any of the perforations by the suspended fibers in the stock screening apparatus of the type of the present invention will most often be used just ahead of a paper making machine and it is anticipated that during the preliminary preparation of that stock at least the majority of heavy reject constituents will be eliminated however it is to.