In the pump chamber of roots pump, there are two "8" shaped rotors mounted perpendicularly to each other on a pair of parallel shafts, and these two rotors are drove by a pair of gears (drive ratio is 1) to make synchronous rotation movement in opposite direction. There are certain clearances between the two rotors, as well as between the rotor and the inner wall of the pump casing, which can realize high-speed operation. As the roots pump has no internal compression and its compression ratio is usually very low. So, high and medium vacuum pumps require a backing pump. Except the structure and manufacturing accuracy of the pump itself, the ultimate vacuum of a roots pump also depends on the ultimate vacuum of the backing pump. In order to increase the ultimate vacuum of the pump, roots pumps can be used in series.
The working principle of the roots pump is similar to that of the roots blower. Due to the continuous rotation of the rotor, the pumped gas enters into the space v0 between the rotor and the pump shell from the intake port, and it is exhausted through the exhaust port. Since the v0 space is completely closed after inhalation, there is no compression and expansion of the gas in the pump chamber. When the top of the rotor turns to the edge of the exhaust port, the V0 space is connected with the exhaust side, a part of the gas returned to the space V0 due to the higher gas pressure on the exhaust side, so that the gas pressure suddenly increases. As the rotor continues to rotate, the gas is discharged out of the pump.