Pulp screens

Claim 1

A PULP SCREEN COMPRISING A CASING, A SCREEN ELEMENT MOUNTED IN THE CASING, MEANS FOR DISTRIBUTING UNSCREENED STOCK AT HIGH VELOCITY ALONG ONE SIDE OF THE SCREEN ELEMENT, A SYSTEM OF VANES MOUNTED FOR MOVEMENT IN THE UNSCREENED STOCK ADJACENT TO THE SCREEN ELEMENT, RESTRAINING MEANS ACTING ON THE SYSTEM OF VANES, AN OUTLET FOR STOCK THAT HAS PASSED THROUGHT THE SCREEN ELEMENT, AND AN OUTLET FOR STOCK THAT HAS FAILED TO PASS THROUGH THE SCREEN ELEMENT; SAID SYSTEM OF VANES BEING DRIVEN ONLY BY THE DRAG EXERTED BY THE HIGH VELOCITY FLOW OF UNSCREENED STOCK BUT RESTRAINED BY SAID RESTRAINING MEANS FOR ACQUIRING A VELOCITY AS HIGH AS THAT OF THE STOCK, EACH VANE BEING SHAPED AND LOCATED TO DEFINE BETWEEN ITSELF AND THE SCREEN ELEMENT A CLEARANCE PASSAGE OF THE SHAPE OF A VENTURI PASSAGE HAVING A SMOOTHLY CONVERGING END AND A SLOWLY DIVERGING END, THE SLOWLY DIVERGING END BEING DEFINED BY THE PORTION OF THE BLADE THAT LEADS IN RESPECT OF ITS DIRECTION OF TRAVEL.