Most rotordynamic analyses typically ignore the potential for critical speeds to be created by traversing a backward precessional whirl mode. While not commonly recognized, a backward mode can be excited using unbalance as the driving force. Based on the analysis of a Jeffcott rotor-bearing model, it was found that the condition for this response to occur is strongly dependent on stiffness asymmetry in the rotordynamic coefficients at the supports. To illustrate the application of this result, a rotordynamic analysis on actual hardware is presented, in which the unbalance excited backward mode resonance is calculated to occur. Test data is also given indicating the presence of the predicted critical speed. It is important to note that although the resonance is due to the backward mode, the precessional direction is forward. Several recommendations are offered with respect to rotor-bearing design so that this unique critical speed situation may be avoided.

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