defining rotor-stator clearance of viscous molecular vacuum pump by numerical methods

The rotor-stator clearance of viscous molecular vacuum pump in the static state is determined by manufacturing process of the pump components and its assembly technique. When a pump rotor starts to have the operating speed a static clearance is transformed into dynamic one. A dynamic clearance is considerably less than the static one owing to increasing pump rotor diameter as a result of centrifugal forces and thermal expansion of material. Moreover, a dynamic clearance non-evenly changes along the length of the vacuum pump wet end depending on the rotor design. To simplify the model under consideration, as an object of study, a single-stage viscous molecular vacuum pump is selected, as its working part is a cylinder. To determine the effect of centrifugal forces and thermal expansion of material is quite simple, as there are a number of techniques allowing calculation. A number of problems arise when there are channels on the outer surface of the rotor, since a changing diameter of the smooth cylinder substantially differs from the changing diameter of the cylinder with the channels.

The set problem is solved by numerical methods using the software package SolidWorks Simulation. To use the SolidWorks Simulation software, an analytical calculation of increasing rotor diameter has been compared with and the numerical one. The obtained results are in agreement within a permissible accuracy of 15%. Therefore, this software package may be used to solve the problem with the cylinder, having channels on the outer surface. The paper presents results of changing diameter of rotor with rectangular channels. Though, channels of the other profiles depending on the required pump performance data are possible at the pump-wet end.

The calculated rotor surface temperatures change from 300 K to 500 K, and rotating speeds vary within 0 to 7000 rev/min. Carbon steel is used as a material, but here the use of aluminum alloys is supposed. The results obtained allow us to estimate a dynamic clearance size at the viscous molecular vacuum pump-wet end under operation conditions. Creating a methodology for determining the clearance size is important for these pumps, since its value has a significant impact on the pump performance data.