Abstract: A rotary blood pump comprises an impeller in a pump housing with a pumping chamber between first and second walls. The impeller operates in a levitated position spaced from the first and second walls in response to hydrodynamic forces which are boosted by hydrodynamic bearing features in the walls. At least one of the impeller or the walls includes at least one mechanical thrust bearing extending between the impeller and each of the walls, wherein the mechanical thrust bearing is configured such that when the impeller is not being held in the levitated position by the hydrodynamic forces then the mechanical thrust bearing is engaged to maintain a predetermined separation between the hydrodynamic bearing features and the impeller. The mechanical thrust bearing is configured such that when the impeller is being held in the levitated position by the hydrodynamic forces then the mechanical thrust bearing is unengaged.