The rotor response to unbalance is calculated using all the bearing stiffness and damping coefficients, which are generated for each specified speed with the specified allowable unbalance. Gyroscopic effects are also included.
The rotor vibration in the horizontal and vertical directions are calculated along with the major ellipse axis (orbit) versus speed. The phase relation at each mass relative to the input unbalance is also calculated. The lateral vibration responses can be plotted for any speed range and are typically plotted for speeds beyond the trip speed. These plots indicate the rotor critical speeds and vibration sensitivity to possible residual unbalances.
Since calculated lateral vibration responses include the stiffness and damping effects of the bearings and cross-coupling, the predicted lateral critical speeds are often higher than the undamped critical speeds indicated by the intersections of bearing stiffnesses shown on the critical speed map.
The unbalance used in the analysis is normally that specified by API. Unbalance is usually simulated at the coupling and near the rotor midspan. Combinations of out-of-phase unbalances can be used to excite conical whirl type modes. The vibration amplitudes can be calculated and plotted for any location on the shaft. The calculated vibration at the bearings can be compared to API allowable.
