Single-stage and multi-stage Roots pumps belong to the category of dry-running rotary displacement vacuum pumps. They are also termed Roots pumps or Roots blowers. The multi-stage Roots pumps described in this chapter belong to the ACP range (adixen clean pump).
In a pump, two synchronously counter-rotating rotors (1) rotate contactlessly in a single housing (Figure 4.9). The rotors have a figure-eight configuration and are separated from one another by a narrow gap. Four to six pairs of rotors are located on the rotor shafts. Each rotor cavity is separated from the others by stator disks with a gas orifice. The gas conveyed is pumped from the inlet port (3) to the outlet port (4).
One shaft is driven by a motor (5). The other shaft is synchronized by a timing gear (6) in the gear box. Lubrication is limited to the gear box and encapsulated low pressure and high pressure bearings (9), which are sealed off from the suction chamber (7) by shaft seals (8).
Because there is no friction in the suction chamber, a Roots vacuum pump can be operated at high rotation speeds of up to 6,000 rpm. The symmetrical distribution of the rotor mass around the shaft axis also results in perfect dynamic balancing, which means that the pump operates extremely quietly in spite of its high speeds.
