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Working principle and structural characteristics of Roots pump

The Roots pump is a positive displacement vacuum pump with two rotors. There are two symmetrically shaped rotors in the pump chamber, and the rotor shapes include two, three, and four lobes. The two rotors rotate in opposite directions to each other and are driven by the shaft end gear to rotate synchronously. There is no contact between the rotors and the wall of the pump chamber. There is usually a gap of 0.15~1.0mm between them, and the pump chamber is sealed by the gap.

Because there is no friction in the pump chamber of the Roots pump, the rotor can run at high speed, generally 1500~3000r/min, and there is no need to use lubricating oil, which can realize the oil-free and clean pumping process. The lubricating parts of the pump are limited to bearings and gears, as well as 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 can obtain a large pumping speed with a small size.

At present, most of the Roots pumps at home and abroad are of horizontal structure. The air inlet of the pump is on the top and the exhaust port is 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 relatively synchronous rotation through a pair of high-precision gears.

The transmission of the pump is mostly direct-coupled, and the large pump is driven by a belt. The motor and transmission gear are mostly located on both sides of the rotor shaft, which is easy to install and disassemble. The torque transmitted by the drive shaft is relatively large, the shaft must have sufficient strength and rigidity, and the shaft and rotor must be solidly consolidated. The sealing of the Roots pump is very important. There are dynamic seals between the overhanging part of the drive shaft, the bearings of the two rotors and the pump chamber, and static seals between the pump chamber and each end cover.

In order to avoid misoperation of the Roots pump, a bypass relief valve is generally provided. According to the special needs of the Roots pump, there are also oil gearboxes, bearings and other components that are separated from the pump chamber.

The heat generated during the compression process of the Roots pump is transferred to the rotor and the pump body. It is difficult for the rotor to transfer heat to the outside of the pump, and the heat of the pump body is easily lost to the surrounding atmosphere. Thus there is a temperature difference between the rotor and the pump body. Exacerbated the thermal expansion of the rotor. When the pump load increases, the rotor expands and the gap disappears, and it is stuck. Therefore, an air-cooled Roots pump appeared, which uses cooled gas or the 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 work under the condition of high pressure difference and high compression ratio. The structure of this pump is simple, and the cooler is connected with the outlet of the pump, 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 volumetric efficiency of the pump.

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