Multi-stage vacuum pump with improved low vacuum pressure performance

A multi-stage vacuum pump 200 comprises a stator assembly defining at least two chambers 100a-f and a rotor assembly 110 having at least two rotors 112a-f housed in respective chambers to define at least two pump stages, including at least an inlet stage 106 and an outlet stage 108. The rotor assembly also includes a shaft 114 supported on a first, axially moveable, bearing 120, and a second, fixed, bearing 130. The inlet stage 106 is positioned closer to the fixed bearing than at least one of the other pump stages. Alternatively, a set axial clearance Ca for the rotor assembly is defined between the set side wall 105a and the rotor 112a of the inlet stage. These embodiments permit tighter control of clearances in the inlet stage to improve lower or "rough" vacuum pressure performance.