Roots pump is a double-rotor positive displacement vacuum pump. There are two symmetrical rotors in the pump cavity, and the rotors are two-lobed, three-lobed and four-lobed. The two rotors rotate in opposite directions and are driven by the shaft end gear to rotate synchronously. The rotors are not in contact with each other, and the rotor and the pump cavity wall are also not in contact. There is usually a gap of 0.15 to 1.0 mm in between, and the pump cavity is sealed by the gap.

Because there is no friction in the pump cavity of the Roots pump, the rotor can run at a high speed, usually 1500~3000r/min, and no lubricating oil is needed, which can realize the oil-free and clean air pumping process. The lubrication parts of the pump are limited to bearings, gears, and dynamic seals. The pump has no reciprocating parts, so it can achieve good dynamic balance. Therefore, the Roots pump runs smoothly, has a high speed, and a small size to obtain a large pumping speed.

At present, most Roots pumps at home and abroad are of horizontal structure, with the pump's air inlet on the top and exhaust on the bottom. This horizontal structure has a low center of gravity and good stability during high-speed operation. The two rotors of the Roots pump realize their relative synchronous rotation through a pair of high-precision gears.

The transmission of pumps is mostly direct-coupled, while large pumps are driven by belts. The motor and transmission gear are mostly arranged on both sides of the rotor shaft, which is convenient for installation and disassembly. The torque transmitted by the driving shaft is relatively large, the shaft must have sufficient strength and rigidity, and the shaft and the rotor must be firmly consolidated. The seal of the Roots pump is very important. There are dynamic seals between the extension part of the driving shaft, the bearings of the two rotors and the pump cavity, and the pump cavity and each end cover are equipped with static seals.

In order to avoid misoperation of Roots pumps, bypass relief valves are generally provided. According to the special needs of Roots pumps, the oil gearbox, bearings and other parts and the pump cavity are also made into a separate structure.

The heat generated during the compression process of the Roots pump is transferred to the rotor and pump body. It is difficult for the rotor to transfer heat to the outside of the pump, and the heat from the pump body is easily lost to the surrounding atmosphere. Therefore, there is a temperature difference between the rotor and the pump body. Increase the thermal expansion of the rotor. When the pump load increases, the rotor expansion will disappear the gap and be stuck. Therefore, an air-cooled Roots pump appears, which uses cooled gas or atmosphere to directly cool the hot rotor, thereby reducing the temperature difference between the rotor and the pump casing and improving the heat resistance of the Roots pump.

This air-cooled Roots pump can be used under high pressure difference and high compression ratio. The pump has a simple structure and the cooler is connected to the pump outlet, which is convenient for maintenance and replacement. Due to the uniform heat dissipation, the gap between the rotor and the pump body can still be kept small, which is beneficial to the improvement of the pump volume efficiency.