Potential vibration and reliability problems with rotating equipment can be prevented by performing lateral critical speed and stability analyses of the rotor. Recommended guidelines for rotordynamic characteristics are given in specifications such as The American Petroleum Institute (API) Standards. These guidelines include definitions of terms, recommended separation margins from critical speeds, allowable shaft vibration, and allowable residual unbalances.
The first step required for calculating lateral critical speeds is to generate an accurate shaft model, which considers each significant section length and diameter and includes all concentrated masses and added inertias. The effective stiffness and damping at the bearings must also be accurately represented over the full clearance range. Variation in load should be considered for bearings in gearboxes. For large rotors with pedestal supports, such as fans and motors, the total stiffness to ground must be included. This includes the oil film, bearing housing, bearing pedestal, and the foundation stiffnesses.
After the shaft has been modeled, the undamped critical speeds are calculated and plotted versus the effective support stiffness at the bearing in a critical speed map. Using the calculated bearing stiffnesses and damping characteristics, the damped rotor response is then calculated for various unbalanced conditions. If the rotor operates at speeds above the first critical speed, the rotor system stability should be checked. Instabilities can occur in systems with low effective damping, often resulting in damage to the seals, labyrinths and bearings.
EDI has developed a general-use rotordynamics code based on system matrix methods. ROTOR-E is used to perform lateral critical speed, unbalance response, and stability analyses. Bearings, seals, aerodynamic load, and support pedestals can be included into the models. The program also contains special elements such as pinned-joints, which can be used to model complex coupled trains. Animated graphics allow the calculated mode shapes and forced vibration response of a rotor to be easily visualized.
EDI provides each of their clients with a detailed technical report that fully documents the analysis procedures, results, conclusions, and recommendations. Each report is submitted in printed, bound form as well as in an electronic version using Adobe Acrobat. In addition, the graphical software is included so that the analysis results can be displayed on the client's computer.
