Imbalance Response and Stability Characteristics of a Rotor Supported by Hybrid Air Foil Bearings

Abstract

This article presents the imbalance response of a 4.84-kg rotor of 49 mm in diameter supported by hybrid air foil bearings (HAFBs). In this work, hybrid means hydrostatic injection of pressurized air into the air film. Previous experimental works by the authors on HAFBs with different designs (elastic supports, overall size, number of top foils) considered the load capacity at low speeds and the cooling effectiveness of the hybrid operation. The current article investigates the rotordynamics performance of HAFBs. The HAFB in this work has three top foils with hydrodynamic preload with a clearance distribution similar to traditional three-lobe bearings. The imbalance response in cylindrical mode is presented with different supply pressures (2.67–4.67 bar). Imbalance responses near the critical speeds do not clearly show the effect of increased pressure on the bearing stiffness, natural frequency, and damping around the critical speeds. However, the frequency of subsynchronous vibrations (considered to be the natural frequency of the rotor-bearing system) at high speeds increases slightly with the pressure. The increased supply pressure is more effective in suppressing the subsynchronous vibrations at high speeds rather than affecting the modal stiffness and natural frequencies. Prediction using modal impedance curves calculated from a linear perturbation method slightly overpredicts (4–9%) the measured natural frequencies.

KEY WORDS:

• Hybrid Foil Bearing
• Oil-Free
• Rotordynamics
• Stability

ACKNOWLEDGEMENT

The test bearing was designed and built with the partial support of Texas Higher Education Coordinating Board under grant number 000512-0057-2007. The project was also partially supported by the Korea Institute of Machinery and Materials.

Review led by Luis San Andres

Notes

The definition of micro or mid-size is subjective depending on the application, and the referenced numerical values of power or journal shaft diameter are rather arbitrary according to the authors’ own experiences.

The phase angle at speeds below critical speeds is not necessarily accurate due to the fact that the rotor may not be completely lifted off the bearing and due to limitations of the baseline subtraction theory, which is based on the assumption of linearity of the system.