Impeller for water pump

An impeller for a water pump, comprising:

• a housing case;
• an impeller base fixed to the housing case and provided with a plurality of holes on a virtual inner peripheral circle side and a plurality of elliptical elongated holes on a virtual outer peripheral circle side;
• a vane body provided with a rotary shaft on an inner peripheral side and a rocking shaft on an outer peripheral side;
• a plate cam in which an elongated groove is formed on an outer peripheral side of a disc portion thereof;
• 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.