A power steering system, and more particularly a power-assisted steering system for a vehicle, comprises a lead screw which is driven by a servo motor and is composed of a nut, which is mounted axially non-displaceably in a frame, and an axially displaceable component, the thread of which is engaged with a thread of the nut, and further comprising a belt pulley, which is non-rotatably connected to the nut and forms part of a traction mechanism gear between the servo motor and the nut, wherein the nut is mounted in the frame by a rolling bearing. So as to provide a power steering system which produces little noise and is easy to install, very precise in operation and also cost-effective to produce, the belt pulley is produced from plastic.