Structural Stiffness, Dry-Friction Coefficient and Equivalent Viscous Damping in a Bump-Type Foil Gas Bearing

Abstract

High performance oil-free turbomachinery implements gas foil bearings (FBs) to improve mechanical efficiency in compact units. FB design, however, is still largely empirical due to their mechanical complexity. The paper provides test results for the structural parameters in a bump-type foil bearing. The stiffness and damping (Coulomb or viscous type) coefficients characterize the bearing compliant structure. The test bearing, 38.1 mm in diameter and length, consists of a thin top foil supported on bump-foil strips. A prior investigation identified the stiffness due to static loads. Presently, the test FB is mounted on a non-rotating stiff shaft and a shaker exerts single frequency loads on the bearing. The dynamic tests are conducted at shaft surface temperatures from 25 °C to 75°C. Time and frequency domain methods are implemented to determine the FB parameters from the recorded periodic load and bearing motions. Both methods deliver identical parameters. The dry friction coefficient ranges from 0.05 to 0.20, increasing as the amplitude of load increases. The recorded motions evidence a resonance at the system natural frequency, i.e. null damping. The test derived equivalent viscous damping is inversely proportional to the motion amplitude and excitation frequency. The characteristic stick-slip of dry friction is dominant at small amplitude dynamic loads leading to a hardening effect (stiffening) of the FB structure. The operating temperature produces shaft growth generating a bearing preload. However, the temperature does not affect significantly the identified FB parameters, albeit the experimental range was too small considering the bearings intended use in industry.

Numerical and experimental investigation of bump foil mechanical behaviour

“…Good agreement between the experimental and theoretical results are found when using a coefficient of friction μ = 0.2 for the simulations. This value corresponds well with common values which is typical in the range 0.1 < μ < 0.5 for steel against steel (0.5 in vacuum) and also with the results obtained by e.g [5,12]. For strips with higher number of bumps, higher global stiffness and larger discrepancies with theoretical results are observed.…”

“…This method is based on the assumption of harmonic oscillations which can be hard to obtain in an experimental set-up. Temperature effects was also investigated [12] and found to be negligible. The dry friction coefficient was found to be nearly constant with the excitation frequency but dependent on the load amplitudes.…”

“…The stiffness of these springs was calculated by the analytical expression of Iordanoff [11]. Furthermore, the equivalent damping was determined experimentally, for a given bump geometry, by assuming a one DOF system to which the experimental data was fitted [12,13]. This method is based on the assumption of harmonic oscillations which can be hard to obtain in an experimental set-up.…”

Experimental and theoretical analysis of a rigid rotor supported by air foil bearings

“…This is especially evident for the direct damping coefficient in the y direction, and generally at lower frequencies. It is important to highlight, that the choice of the structural loss factor η = 0.25 is based partly on a guess and partly on previous investigations [37,45]. However, the equivalent structural loss factor is strongly dependant on both frequency and displacement amplitude.…”

Experimental study on multi-decked protuberant foil thrust bearing with different number of thrust pads

“…Up to now, there were a number of literatures on journal bearing concerning load capacity, stability, etc. There have been benchmarked design and test rigs for journal bearing (DellaCorte et al, 2008) (Rudloff et al, 2011) (Dellacorte, 1998) (Howard et al, 2001a) (Rubio and Andres et al, 2006). Comparing to foil journal bearing, the thrust bearing did not enjoy the same interest and development.…”