Page 125 - Improving Machinery Reliability

Machinery Reliubility Audits and Reviews 97

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Figure 3-1 1. Rotor response calculations for midspan unbalance and minimum clearance.

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Figure 3-1 2. Rotor response calculations for midspan unbalance and maximum clearance.

Sensitivity to Pedestal and Foundation Flexibility. The stiffness, mass, and
damping of the bearing support structure should be considered in a rotordynamic
audit. Basically, the bearing and pedestal stiffnesses combine as springs in series. If
the bearing stiffnesses are very low compared to the pedestal stiffnesses, the critical
speeds and rotor response will not be greatly affected by the pedestal and support
flexibility. Computer programs for dual level rotors are available that can consider
the pedestal mass, stiffness, and damping values in the rotor response to unbalance
calculations.