1. IN A ROTARY PUMP HAVING A CASING DEFINING A GENERALLY CYLINDRICAL CHAMBER HAVING A PERIPHERAL WALL AND PARALLEL OPPOSED END WALLS, AND A ROTOR HAVING A RADIALLY SLOTTED PORTION ECCENTRICALLY MOUNTED THEREIN AND ADAPTED TO HAVE AN IMPELLER ROLLER MOUNTED IN THE SLOTTED PORTION THEREOF, THE IMPROVEMENT WHICH COMPRISES AN IMPELLER ROLLER DISPOSED IN SAID ROTOR FOR ROLLING ENGAGEMENT ON SAID PERIPHERAL WALL AND SLIDING ENGAGEMENT ON SAID END WALLS AND INCLUDING A CYLINDRICAL CORE OF A HIGH SPECIFIC GRAVITY MATERIAL HAVING PARALLEL FLAT ENDS NORMAL TO THE AXIS, AND A CYLINDRICAL PLASTIC JACKET OF A LOWER SPECIFIC GRAVITY MATERIAL ENCLOSING SAID CORE TO UNIFORM THICKNESS PERIPHERALLY AND TO UNIFORM AND LIKE THICKNESS ON OPPOSITE ENDS, SAID CORE HAVING AN AXIAL THROUGH HOLE OF THE SAME DIAMETER END TO END PROVIDED THEREIN AND BEING OF UNIFORM DIAMETER AND WEIGHT FROM END TO END AND BEING ADAPTED TO BE ACCURATELY CENTERED BY MEANS OF SAID AXIAL HOLE AT BOTH ENDS IN THE JACKET WHICH IS ALSO OF UNIFORM DIAMETER AND EQUAL WEIGHT AT BOTH ENDS AND HAS CENTER HOLES OF THE SAME SIZE PROVIDED THROUGH BOTH ENDS COMMUNICATING WITH THE AXIAL THROUGH HOLE IN THE CORE, WHEREBY THE IMPELLER ROLLER IS OF EVEN WEIGHT AT BOTH ENDS AND IS ADAPTED TO ENGAGE WITH PRECISELY THE SAME BEARING PRESSURE PHERIPHERALLY AT BOTH ENDS THE INSIDE OF THE PERIPHERAL WALL OF SAID PUMP CHAMBER ON WHICH IT HAS ROLLING ENGAGEMENT ITS FULL LENGTH THE FULL WIDTH OF SAID CHAMBER, THE PLASTIC COVERED FLAT ENDS OF SAID IMPELLER ROLLER ALSO SLIDABLY ENGAGING THE END WALLS OF SAID PUMP CHAMBER WITH EVENLY BALANCED PRESSURE BY VIRTUE OF THESE ENDS BEING MAINTAINED THROUGH SAID AXIAL HOLE UNDER THE SAME HYDRAULIC PRESSURE IN THE OPERATION OF THE PUMP.