On Tilting Pad Carbon–Graphite Porous Journal Bearings: Measurements of Imbalance Response and Comparison to Predictions of Bearing Performance and System Dynamic Response

Abstract

Microturbomachinery implements gas bearings to produce significant power with high efficiency at high rotor speed operation. Externally pressurized porous (EPP) gas bearings allow nearly friction-free operation. This article presents rotor dynamic tests conducted with a large and heavy rotor (100 mm outer diameter and 285 N weight) supported on a pair of five-pad tilting pad EPP gas bearings. The specific static load on each bearing is 19.5 kPa. Rotor deceleration tests from 18 krpm (94 m/s journal surface speed) take over 800 s to bring the rotor to a full stop; hence, the derived drag friction coefficient is just ∼0.003. The measured rotor responses to calibrated imbalances evidence a critical speed at ∼7.5 krpm and a system damping ratio of ∼11%. A predictive model for the forced performance of tilting pad EPP gas bearings includes the gas flow through the porous substrate and an operating clearance that is a function of the supply pressure into the bearings, the pads’ pivot flexibility, and the assembly preload. Predictions of the rotor–bearing system response to imbalance produce accurate natural frequencies and critical speeds, both a function of the supply pressure, and slightly more damping than the one experimentally derived. Dry friction arising from the spring washers acting as the pivot element on the bearings’ pads may explain the difference between measurements and predictions, in particular for operation with a low magnitude of supply pressure and near the system critical speed.

Keywords:

• Rotordynamics
• gas bearing
• porous surface bearing

Notes

1 The bearing number Λ ∼ 3/2 μ Ω (D/c)²/P_a shows the influence of compressibility of bearing performance; Λ → 0 represents a nearly incompressible fluid film bearing.

2 In this article, all pressures are absolute.

3 The organization (flow of information) of the article stresses the experimental procedure and test results while detailing supporting material that may be found in the published literature in the Appendices.

4 The equation of motion for the rotor while it decelerates is $I_P\dot{\Omega }+2C_{\theta }\Omega =0$ with solution $\Omega ={\Omega }_0{e}^{-\raisebox{1ex}{$t$}\!\left/ \!\raisebox{-1ex}{$\tau $}\right.}$. When t = τ, Ω/Ω₀ = 1/e = 0.368.

Additional information

Funding

The author is grateful for the financial support of the TEES Turbomachinery Research Consortium (TRC) and the equipment donation from New Way® Air Bearings. Thanks to graduate assistant Y. Zheng for conducting the hands-on work. TRC supported financially his education and technical work.