Composite dry vacuum pump having roots and screw rotor

A complex dry vacuum pump including a root rotor and a screw rotor, comprising:

• a housing having an interior receiving space, a suction opening at one side of the housing, and a discharge opening at another side of the housing;
• first and second root rotors which are located in the interior receiving space of the housing and are installed in such a manner as to be engaged with each other;
• first and second screw rotors which are received in the interior receiving space of the housing and are installed in such a manner as to be engaged with each other adjacent to the first and second root rotors;
• first and second power transmission shafts extending through each center of the first and second root rotors and the first and second screw rotors;
• and a motor which is able to rotate the first and second power transmission shafts;
• wherein the first and second root rotors include three lobes, and
• wherein one lobe among the three lobes of the first root rotor has a length from the center of rotation to the end of the lobe shorter than lengths of the remaining two lobes of the three lobes of the first root rotor, and a part of the second root rotor has a shape that makes contact with the lobe of the first root rotor having a shorter length while the first and second root rotors are being rotated.