Dynamic Analysis Of Molecular Pump Rotor - Bearing System

Dynamic analysis of rotor-bearing systems is an important step in rotary machinery design. High-speed rotary machines always have a working speed that is above the second critical speed, in such cases, severe vibration can be expected when passing critical speeds, which must be controlled. The deployment of dampers at supports can effectively control vibration. This thesis aims at damper application on high-speed molecular pump, investigates the viscoelastic damper’s effect on the dynamic characteristics of the rotor-bearing system. Following studies are included:(1) Constructing finite element model of the molecular pump’s rotor-bearing system. Structure of the molecular pump’s rotor-bearing system is examined, and the matrices of shafts, disk and supports are derived, assemble these matrices and the system’s equations of motion are established.(2) Solving equations of motion. Free and forced vibration analyses are carried out. Campbell graphs are drawn, critical speeds within working speed domain are calculated, first three modal shapes are derived, vibration decrements are calculated, and the system’s stability is validated. Unbalance responses on different locations on the rotor are calculated, and effects of the viscoelastic damper’s elastic stiffness and loss coefficient on critical speeds, unbalance response and other dynamic characteristics, are analyzed. The results show that rotor’s maximum vibrating amplitude is greatly dependent on both elastic stiffness and loss coefficient, while critical speeds are primarily affected by elastic stiffness. The elastic stiffness of supports, meaning permanent magnet bearing (PMB) and viscoelastic support, should not be close to each other to avoid severe vibration.(3) Testing the characteristics of supports. A test rig is constructed, force-displacement characteristics of PMBs and viscoelastic dampers under different conditions are tested, and the supports’elastic stiffness are calculated.(4) Testing the dynamic characteristics of molecular pump’s rotor-bearing system. Another test rig is constructed to conduct this test, rotor’s vibration is analyzed and comparison is made between the test results and the theoretical analyses to validate the latter. The spectrum of the vibration is also analyzed. Rotor axis’s orbits under different rotating speeds are drawn to validate rotor-bearing system’s working status. With the above theoretical and experimental studies, this thesis investigates the effect of support’s characteristics on rotor-bearing system’s dynamic characteristics, and proposed a technique to avoid severe vibration. The studies of this thesis are applicable to the design of molecular pump.