Impeller for water pump

The object of the present invention is to maintain a discharge performance by making a discharge flow rate constant in a low rotation region of an engine rotation speed and to reduce the discharge flow rate in a high rotation region. The impeller comprises: a housing case; an impeller base; a vane body; a plate cam; and a single torsion spring, wherein the torsion spring and the plate cam are housed in the housing case and the impeller base, a plurality of vane bodies are provided on an upper surface of the impeller base, the rotary shaft of vane body is inserted rotatably into the holes in the impeller base, the rocking shaft of the vane body is inserted with play into the elliptical elongated holes in the impeller base and the elongated groove in the plate cam, and the rocking shaft is positioned on an outer peripheral side of the elliptical elongated hole in the impeller base by an elastic force of the torsion spring in an engine low rotation region.