Dry pump for gases, comprising: ▪ a first rotor comprising a first lobe portion and a first screw, ▪ a second rotor comprising a second lobe portion and a second screw, and ▪ a casing in which the first and second rotors (1, 2) are mounted rotating in such a way that the first and second screws (1B, 2B) mesh and the first and second lobe portions (1A, 2A) engage one with the other, the casing delimiting an internal volume in which are located together the first and second screws (1B, 2B) and the first and second lobe portions (1A, 2A), at least one inlet coming out into the internal volume at the first and second lobe portions (1A, 2A), at least one outlet from the internal volume being located opposite the inlet relative to the first and second screws (1B, 2B), characterized in that each of the first and second screws (1B, 2B) comprises a threading invariable along its length, the first and second rotors (1, 2) being rotating in opposite directions in such a way as to be able to be located successively in: - a first configuration in which the first and second lobe portions (1A, 2A), a portion of the first screw, a portion of the second screw and the casing together delimit a chamber which is closed, - a second configuration in which the chamber is always delimited by the first and second lobe portions (1A, 2A), a portion of the first screw, a portion of the second screw and the casing in such a way as to be closed and have a smaller capacity than in the first configuration, - a third configuration in which the chamber is displaced entirely at the first and second screws (1B, 2B) and isolated from the lobe portions at least by a helical thread of the threading of the first screw, a helical thread of the threading of the second screw and a blocking made by an intersection of the helical thread of the first screw and of the helical thread of the second screw, and - a fourth configuration in which the chamber is displaced at the downstream end of the first and second screws (1B, 2B) and is in communication with the outlet. ▪ a first rotor comprising a first lobe portion and a first screw, ▪ a second rotor comprising a second lobe portion and a second screw, and ▪ a casing in which the first and second rotors (1, 2) are mounted rotating in such a way that the first and second screws (1B, 2B) mesh and the first and second lobe portions (1A, 2A) engage one with the other, the casing delimiting an internal volume in which are located together the first and second screws (1B, 2B) and the first and second lobe portions (1A, 2A), at least one inlet coming out into the internal volume at the first and second lobe portions (1A, 2A), at least one outlet from the internal volume being located opposite the inlet relative to the first and second screws (1B, 2B), characterized in that each of the first and second screws (1B, 2B) comprises a threading invariable along its length, the first and second rotors (1, 2) being rotating in opposite directions in such a way as to be able to be located successively in: - a first configuration in which the first and second lobe portions (1A, 2A), a portion of the first screw, a portion of the second screw and the casing together delimit a chamber which is closed, - a second configuration in which the chamber is always delimited by the first and second lobe portions (1A, 2A), a portion of the first screw, a portion of the second screw and the casing in such a way as to be closed and have a smaller capacity than in the first configuration, - a third configuration in which the chamber is displaced entirely at the first and second screws (1B, 2B) and isolated from the lobe portions at least by a helical thread of the threading of the first screw, a helical thread of the threading of the second screw and a blocking made by an intersection of the helical thread of the first screw and of the helical thread of the second screw, and - a fourth configuration in which the chamber is displaced at the downstream end of the first and second screws (1B, 2B) and is in communication with the outlet. - a first configuration in which the first and second lobe portions (1A, 2A), a portion of the first screw, a portion of the second screw and the casing together delimit a chamber which is closed, - a second configuration in which the chamber is always delimited by the first and second lobe portions (1A, 2A), a portion of the first screw, a portion of the second screw and the casing in such a way as to be closed and have a smaller capacity than in the first configuration, - a third configuration in which the chamber is displaced entirely at the first and second screws (1B, 2B) and isolated from the lobe portions at least by a helical thread of the threading of the first screw, a helical thread of the threading of the second screw and a blocking made by an intersection of the helical thread of the first screw and of the helical thread of the second screw, and - a fourth configuration in which the chamber is displaced at the downstream end of the first and second screws (1B, 2B) and is in communication with the outlet.