Roots pump

A roots pump comprising:

• a housing having a suction hole through which a fluid is introduced and a discharge hole through which the fluid is discharged;
• a rotor chamber formed in the housing and connected to the suction hole and the discharge hole;
• a rotary shaft disposed in the housing;
• a rotor having an insertion hole through which the rotary shaft is inserted and a hole-forming surface that forms the insertion hole, the rotor being configured to rotate, in the rotor chamber, together with the rotary shaft press-fitted in the hole-forming surface;
• and
• a first bearing and a second bearing supporting the rotary shaft respectively on opposite sides of the rotor so that the rotary shaft is rotatable relative to the housing, each of the first bearing and the second bearing including:
• an inner ring;
• an outer ring surrounding the inner ring and supported by the housing;
• and
• a rolling element disposed between the inner ring and the outer ring, wherein
• the rotary shaft is press-fitted in the inner ring of the first bearing, and fitted in the inner ring of the second bearing with a clearance between the rotary shaft and the inner ring of the second bearing,
• a coefficient of linear expansion of a material of the housing and a coefficient of linear expansion of a material of the rotor are larger than a coefficient of linear expansion of a material of the rotary shaft, and
• a rotation of the rotor causes the fluid to be introduced to the rotor chamber through the suction hole and discharged from the rotor chamber through the discharge hole, wherein
• an edge portion of the hole-forming surface adjacent to the first bearing serves as a press-fitting portion in which the rotary shaft is press-fitted, and
• at least an edge portion of the hole-forming surface adjacent to the second bearing serves as a clearance-fitting portion in which the rotary shaft is fitted with a clearance between the rotary shaft and the clearance-fitting portion.