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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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Figure 35 Whirl speed map—straddle-mount rotor with pedestal supports.<br />

motion <strong>of</strong> the pedestal masses and small motion <strong>of</strong> the rotating assembly.<br />

The fifth critical speed mode primarily involves bending <strong>of</strong> the rotating<br />

assembly and is essentially identical with the third critical speed mode with<br />

rigid supports.<br />

If the pedestal stiffness kp is pincreased ffiffiffiffiffi by a factor <strong>of</strong> 10, so the pedestal<br />

natural frequency is increased by 10 to 60,393 rpm, the first three forward<br />

critical speeds then become 10,345, 17,343, and 60,440 rpm. Thus, this<br />

higher-frequency (very stiff) pedestal design approaches the equivalent <strong>of</strong> a<br />

rigid support and the critical speeds are only slightly changed from those<br />

with a rigid support.<br />

A foundation model, which includes only lumped pedestal masses at<br />

each bearing location, is useful in many situations; however, it possesses<br />

serious deficiencies. The pedestal mass only allows for the inclusion <strong>of</strong> one<br />

foundation frequency at each bearing location and does not provide for<br />

either static or dynamic coupling between bearing locations. If the true<br />

support structure is quite flexible and includes several natural frequencies<br />

Copyright © 2003 Marcel Dekker, Inc.

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