Gas friction vacuum pump having at least three differently configured pump stages releasably connected together

A gas friction pump having a vacuum side and a pressure side together defining a respective direction of flow of a gas through the pump, and comprising:

• at least three differently configured pump stages sequentially and releasably connected to each other, each comprising a housing section, and a rotor located within the respective housing section;
• a first of said pump stages being a molecular pump stage located on the pressure side of said pump, the rotor of said molecular pump stage having a frustoconical hub tapering away from the pressure side;
• a second of said pump stages being a filling stage preceding said molecular pump stage in a direction toward the vacuum side, the housing section of said filling stage constituting a stator surrounding the rotor of said filling stage, said rotor of said filling stage comprising a frustoconical, central hub tapering away from the pressure side and adjoining the frustoconical hub of said molecular pump stage so that the two adjoining hubs form a continuous, frustoconical shape, said rotor of said filling stage further including a plurality of radial, helical webs attached to the central hub, each helical web having a pitch and a width decreasing in a direction toward the pressure side for the pumping of the gas;
• and
• a third of said pump stages being a turbomolecular pump stage preceding said filling pump stage and being on the vacuum side of said pump.