Abstract
This paper presents a new view of a key overlooked phenomenon when dealing with significantly eccentric rotors, namely the switch of the rotor’s axis of precession and consequent orientation in its bearings while passing through the critical speed region. This occurs in conjunction with torque effects unique to the case where a rotor’s principal mass axis and torque input axis are not coincident. This condition also governs the rotor’s phase shift process. Around the critical speed, the inertia from the eccentric mass becomes sufficiently large as to alter the mode of rotation, bringing the rotor toward a “state of least action”, where the precessional orbit rapidly decreases, and the rotor begins to rotate about its principal mass axis. The most immediate benefit of recognizing this behavior is in the development of a new balancing method pertaining especially to flexible, bowed or eccentric rotors, designed for use in balancing facilities.
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Notes
- 1.
A major OEM developed similar standards in which runout/eccentricity limits are approximately twice those in ISO 1940. By the author’s experience, this is an appropriate and suitable adjustment, as the ISO 1940 limits are more conservative than is necessary in practice.
- 2.
In this case, an accurate determination of an orbit “high point” may be nearly impossible, and the phase angle between this and a shaft mark is not necessarily a reliable measurement.
- 3.
This implies that the root of bearing instabilities, including subsynchronous whirl and oil whip, really arises from the condition of the rotor itself and not bearing design, though clearly with proper bearing type and design, the system can “withstand” and stabilize a much wider breadth of dynamic behavior from rotor imperfections.
- 4.
In the standard classical view, rotor “vibration” is considered acting in the line of the bearing force response, which then can be presented as a spring-mass-damper system, but it is erroneous to assume this visualization as the behavior of the shaft itself.
- 5.
This “amplitude dip” correlates to the “torque moment”-driven flip of the rotor, where the rotor briefly rotates about its principal mass axis in a “least action” state. For equivalent unbalance (oz-in), the dip’s occurrence is dependent on input torque combined with the ratio of eccentricity (distance) to eccentric mass (weight). This dip occurs at pre-critical speed when eccentric mass is large but its distance from the geometric axis is small, and at post-critical speed when the total eccentric mass is smaller but at a larger distance [5].
References
Racic Z (2011) Rotordynamics and balancing revisited. In: Vibration institute proceedings: 35th annual meeting, San Antonio, TX
Racic Z, Racic M (2014) Development of a new balancing approach for significantly eccentric or bowed rotors. In: Paolo Pennacchi (ed) Proceedings of the 9th IFToMM International Conference on Rotor Dynamics, vol. 21. Milan, Italy. Mechanisms and Machine Science, 22–25 Sept 2014
Chen WJ, Edgar J (2005) Gunter, introduction to dynamics of rotor-bearing systems, eigen technologies. RODYN Vibration Analysis Inc., Charlottesville
John M (1988) Vance: rotordynamics of turbomachinery. Wiley, New York
Gunter EJ, Barrett LE, Allaire PE (1976) Balancing of multimass flexible rotors, part I: theory and part II: experimental results. In: Proceedings of the fifth turbomachinery symposium, Texas A&M, Oct 1976
Zhyvotov AY (2011) Resonance effect, critical and resonance velocities. In: 13th world congress in mechanism and machine science, Guanajuato, Mexico, 19–25 June 2011
Acknowledgments
This paper was developed with the aid of the concept and experimental results presented by A.Y. Zhyvotov in his Paper: “Resonance Effect, Critical and Resonance Velocities” [6].
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© 2015 Springer International Publishing Switzerland
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Racic, Z., Racic, M. (2015). Behavior of Eccentric Rotors Through the Critical Speed Range. In: Pennacchi, P. (eds) Proceedings of the 9th IFToMM International Conference on Rotor Dynamics. Mechanisms and Machine Science, vol 21. Springer, Cham. https://doi.org/10.1007/978-3-319-06590-8_20
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DOI: https://doi.org/10.1007/978-3-319-06590-8_20
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