Modeling of Rotor Shafting for Lower Mode Accuracy: Influence of Section L/D

The transfer matrix and finite element methods are currently widely used to model rotor shafting for calculation of critical speeds, forced response, and stability. For many rotors, long uniform shafting sections occur on the ends of the rotors which could be treated without consideration of section L/D. The use of finite element continuous mass models makes it possible to consider longer sections than previously considered using lumped inertia models. This paper will review a major mass reduction technique which has been used for transfer matrix lumped mass models and compares the results to those obtained by the continuous mass finite element method. The required section L/D for desired accuracy of the lowest eight modes of a uniform shaft (4 forward, 4 backward) will be studied for both simple property splitting and a modified major mass reduction technique. The necessary section L/D for desired accuracy for varying bearing to shaft stiffness ratios will be presented. A comparison of the finite element method required section L/D to that of the transfer matrix major mass method is presented for both a uniform shafting geometry and a typical centrifugal compressor model. Conclusions and recommendations will be given concerning the required section L/D for improved rotor system analysis accuracy when using lumped inertia rotor models.