Analysis of elastic supports and rotor flexibility for dynamics of a cantilever impeller

Abstract

Turbochargers of modern internal combustion engines experience severe service conditions. In particular the compressor impeller is driven at extremely high angular velocity (30,000 rpm and more). Furthermore the increased requirements for the air intake dictate ever larger dimensions of the rotor disk up to 250 mm. These two factors lead to a pressing problem of stiffness, strength and dynamic stability of the newly designed units. This problem is analyzed with the help of a newly created parametric model of cantilever impeller rotor dynamics. This model accounts for the flexibility of the drive shaft and the impeller disk, quasistatic inertial and aerodynamic loads. The rotordynamics analysis is performed with a specific focus on the support behavior. The effect of initial gap and resilient characteristics of bearings and bushings is taken into account. As a result various components of displacements can be evaluated for different values of the disk diameter and angular velocity. It is critical for the design that neither radial nor axial displacement should not exceed given bounds. Furthermore, critical speeds of the studied rotor system have been determined for the various configurations of the disk and shaft dimensions. The proposed solution is to lower the first one or two eigenfrequencies of the system below the operating domain, so that the rotor becomes stable to any external excitations. Altogether, the developed mechanical analysis tool has proved itself as a useful tool for the engineering practice that allows to eliminate fatal flaws at early design stages.