Among the factors leading to destructive instability of rotors in high-speed, high-performance turbomachinery is excessive rubbing between rotating and stationary parts. Although in many designs this is intentional, such as tight-fitting turbine tip seals, uncontrolled contact can be disastrous. This paper proposes a mathematical rationale of rubbing identification; defines the limits between benign contact and the initiation of a destructive instability based on harmonic spectral data; correlates the results with industry experience, published findings, and extensive laboratory tests; and details the response format of diagnostic data in actual cases. This detection methodology aids in making practical testing decisions based on a reliable indication of the operating condition. Criteria are presented to infer rubbing related to simple unbalanced response and to differentiate it from bearing deadband effects.

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