Critical speed analysis and resonance margin promoting for one helium turbine rotor

The evaluation of natural vibration characteristics is vital in the design of high-speed rotating machinery. One three-dimensional solid finite element model describing the complex configuration of a helium turbine rotor is established. The effects of the nonlinear stiffness caused by the tilting pad bearings are considered under a nominal clearance 0.2%, preload rate 0.25, and 0.5, respectively. Taking into account the bearing nonlinearity, the evaluation procedure of the rotor critical speed is discussed and the error analysis is carried out. Moreover, the dynamical effects of the generator rotor are investigated which is connected to the helium turbine rotor by one flexible coupling. Three aspects are highlighted. The first is the modeling for the blades. The blades are modeled as an equivalent ring encircling the disk which have the same mass and moment of inertia, avoiding dispensable local vibration modes such as blade-disk coupling vibration. The second is the consideration of the nonlinear stiffness and damping caused by the sliding bearing during critical speed analysis. The variation of the equivalent stiffness and damping of sliding bearings with preload rate, nominal clearance, and oil pressure is equated as a function of rotating speed and applied to the calculation of eigenvalues. Finally, the article presents the analysis of the effects of the generator rotor and diaphragm coupling on the dynamic characteristics of the helium turbine rotor. The results indicate that the generator rotor and diaphragm coupling have little effect on the first five modal shapes of helium turbine rotor, but the fourth and fifth modal frequencies decrease by 20% and 14%, respectively. The decreasing ismainly influenced by the additional mass of diaphragm coupling but not the additional stiffness. The resonance margin can be broadened by increasing the mass of the diaphragm coupling.