Non-linear Behavior and Loading Capability of the Spring Support in Lateral Response

Abstract

This paper investigates the dynamic behavior of a rotor—bearing system supported by elastic springs. The system itself is made of several Belleville washer springs located circumferentially around the bearing. The standard accepted design criteria for such system are based on the estimation of the stiffness from analytical formulas developed in 1930s under the assumption of radial deformation only on the springs. Theoretically this system should generate low stiffness in comparison to bearing and pedestal stiffness. However the author experience showed that secondary stiffness effects become dominant and introduce additional non-linear stiffness component, what changes significantly lateral response of the system. First, a classic approach based on an analytical 1D solution for radial stiffness is demonstrated with comparison to simulations including also elasto–plastic material behavior. Then the 3D model is introduced to estimate combined radial and shear stiffness. Here the most important finding is that up to now neglected shear stiffness is much higher than the radial one. Also FEA simulations showed high variability of the actual value of the spring stiffness suggesting overall non-linearity and possible hysteresis under cyclic loading. A linearization procedure is outlined in the paper and presented for an example spring configuration from a gas turbine installation. Eventually the combined radial and shear stiffness is implemented in a PGT25 gas turbine model. The real rotor testing in the bunker revealed that the usually neglected shear stiffness of the Belleville washer springs has significantly more influence on vibration profile, than compared to simulation assuming only radial stiffness.