The invention provides a rotor (5) of the vacuum pump molecular engine comprising an upstream rotor segment (5α) type turbo of metal or alloy, and a downstream portion of the rotor (5c) of the holweck type of composite material. The downstream section of the rotor (5c) comprises a reinforcing structure of long fibers distributed in a manner that is variable as a function of the section in question: in the annular connection region (5d) with the upstream rotor segment (5a), the fibers are inclined and / or spaced apart, in order to maintain the composite material has a flexibility allowing it to deform in order to follow the deformations of the metal of the upstream rotor segment (5a) in operation; by contrast, the fibers are clamped together and form of the contiguous turns in the downstream region of the skirt (5e), with a greater rigidity to withstand mechanical stresses during the rapid rotation of the rotor in operation. It is thus possible to carry out of the rotors in which the skirt of the holweck type has a diameter which is increased in order to improve the properties of the pump.
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