A screw pump comprises a body which has a cylindrical duct formed therein. A screw shaft is located in the cylindrical duct and is arranged to transport liquid between an inlet and an outlet of the screw pump. The screw shaft has a screw thread which defines a helical liquid path within the cylindrical duct. The body of the screw pump includes a groove extending along an inside surface of the cylindrical duct, the groove defining, in part, a ball-passage which extends alongside the screw shaft and intersects with the helical liquid path. The ball-passage contains a plurality of balls which are guided along the ball-passage by the screw thread when the screw shaft rotates relative to the body.