Rotordynamic Performance of Flexure Pivot Hydrostatic Gas Bearings for Oil-Free Turbomachinery

Abstract

Micro-turbomachinery demands gas bearings to ensure compactness, light weight, and extreme temperature operation. Gas bearings with large stiffness and damping, and preferably of low cost, will enable successful commercial applications. Presently, tests conducted on a small rotor supported on flexure pivot hydrostatic pad gas bearings (FPTPBs) demonstrate stable rotordynamic responses up to 100,000rpm (limit of the drive motor). Test rotor responses show the feed pressure raises the system critical speed (increase in bearing direct stiffness) while the viscous damping ratio decreases. Predictions correlate favorably with experimentally identified (synchronous) direct stiffness bearing force coefficients. Identified experimental gas bearing synchronous damping coefficients are 50% or less of the predicted magnitudes, though remaining relatively constant as the rotor speed increases. Tests without feed pressure show the rotor becomes unstable at ∼81krpm with a whirl frequency ratio of 20%. FPTPBs are mechanically complex and more expensive than cylindrical plain bearings. However, their enhanced stability characteristics and predictable rotordynamic performance makes them desirable for the envisioned oil-free applications in high speed micro-turbomachinery.

Nonlinear Numerical Prediction of a Rotor–Bearing System Using Damped Flexure Pivot Tilting Pad Gas Bearings

“…[insert Figure 1] FPTPGBs and its improved structure have been extensively investigated because of their particular advantages over traditional tilting pad gas bearings (5, [13][14][15][17][18][19][20]. San Andrés (13) built a computational model, which considering the external pressurisation, to predict the static and dynamic performance of hybrid FPTPGBs.…”

“…Moreover, the assembly procedure is costly and time consuming, which increase the fabrication cost. Thus, bearings with thin flexure webs (flexure pivot tilting pad gas bearings [FPTPGBs]) connecting the pads and bearing housing were proposed (11)(12)(13)(14). FPTPGBs have low cross-coupled stiffness because the webs can be fabricated to extremely low stiffness.…”

Limits of Dynamic Coefficients of Self-Acting Tilting-Pad Gas Bearings

“…[13] into Eq. [12],δ 0 j can be obtained: [14] When the journal and the pads perturb with the same frequency , the dimensionless dynamic gas-film forces of the full tilting-pad bearing can be written as…”

“…Yu,et al (14) and Yang,et al (15) employ the partial derivative method to calculated the dynamic coefficients of gas bearings and mainly investigate the effects of the perturbation frequency on dynamic coefficients. Zhu and San Andres (12) and Yang, et al (15) show that the dynamic characteristics of the tilting-pad gas bearing are not only dependent on the perturbation frequency of the journal but also on the perturbation frequency of their pads.…”

“…They also show the strong effects of excitation frequency on the dynamic coefficients. Zhu and San Andres (12) conduct the experiments and also use the method presented in San Andres (13) to predict the static and dynamic performance of flexure pivot tilting-pad bearings. The numerical results indicate the dependency of dynamic characteristics of the bearing on the perturbation frequency.…”