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.