A dry pump for gases comprises a first rotor (1) comprising a first lobe portion (1A) and a first screw (1B), as well as a second rotor (2) comprising a second lobe portion (2A) and a second screw (2B). A casing delimits an internal volume in which are located together the first and second screws (1B, 2B) and the first and second lobe portions (1A, 2A). Each of the first and second screws (1B, 2B) comprises a threading invariable along its length. The first and second rotors (1, 2) turn in opposite directions and are located in successive configurations. In a first configuration of the rotors, the first and second lobe portions (1A, 2A), a portion of the first screw (1B), a portion of the second screw (2B) and the casing together delimit a chamber (30) which is closed. In a second configuration of the rotors, the chamber (30) has a smaller capacity than in the first configuration. In a third configuration of the rotors, the chamber (30) is displaced entirely at the first and second screws (1B, 2B) and isolated from the lobe portions.