| 摘要 |
An oil-free two-stage vacuum pump is disclosed, which comprises a first and a second pump stage 200A, 200 B coupled for series driving. The first and second pump stages have their discharge spaces 56, 57 capable of being communicated with each other via a bypass passage. The bypass passage is provided with a pressure control valve 125 to be closed when the pressure in it becomes lower than a predetermined pressure. This permits scroll size reduction. In addition, the pump is free from such problems posed in the case of large scroll size as vibrations of drive shaft due to warping thereof in high speed rotation and noise and heat generation or durability reduction due to such cause as non-uniform contact between stationary and revolving scrolls. Moreover, in the compression step in the first pump stage 200A, gas withdrawn is under high pressure because the gas pressure in a sealed vessel to be evacuated is close to the atmospheric pressure in an initial stage of driving. With the pressure increase beyond a predetermined pressure, a pressure control valve 125 is opened, so that the compressed gas under high pressure is no longer supplied to the second pump stage 200B but is exhausted to the outside 58. The second pump stage thus does not withdraw compressed gas under pressure higher than the atmospheric pressure, and is free from heat generation that might result from excessive compression. <IMAGE> |
