Bump-Type Foil Bearing Structural Stiffness: Experiments and Predictions

Abstract

Gas foil bearings (FB) satisfy many of the requirements noted for novel oil-free turbomachinery. However, FB design remains largely empirical, in spite of successful commercial applications. The mechanical structural characteristics of foil bearings, namely stiffness and damping, have been largely ignored in the archival literature. Four commercial bump-type foil bearings were acquired to measure their load capacity under conditions of no shaft rotation. The test bearings contain a single Teflon coated foil supported on 25 bumps. The nominal radial clearance is 0.036 mm for a 38 mm journal. A simple test set up was assembled to measure the FB deflections resulting from static loads. The tests were conducted with three shafts of increasing diameter to induce a degree of preload into the FB structure. Static measurements show nonlinear FB deflections, varying with the orientation of the load relative to the foil spot weld. Loading and unloading tests evidence hysteresis. The FB structural stiffness increases as the bumps-foil radial deflection increases (hardening effect). The assembly preload results in notable stiffness changes, in particular for small radial loads. A simple analytical model assembles individual bump stiffnesses and renders predictions for the FB structural stiffness as a function of the bump geometry and material, dry-friction coefficient, load orientation, clearance and preload. The model predicts well the test data, including the hardening effect. The uncertainty in the actual clearance (gap) upon assembly of a shaft into a FB affects most the predictions.

Structural Properties of Foil Bearings: A Closed-Form Solution Validated with Finite Element Analysis

“…It is therefore of paramount importance to gain a good understanding of the structural response of a foil bearing to the hydrodynamic pressure developed in the lubricating film. First attempts to model the stiffness of a foil bearing did not take into consideration the interaction between bumps in the same bump strip and modeled each bump as an independent spring (Heshmat,et al (3); Iordanoff (4); Rubio and San Andres (5)). These models are based on the elastic deflection of a curved beam and are very easy to use, although neglecting the interactions between bumps may cause significant errors.…”

Identification of Structural Stiffness and Energy Dissipation Parameters in a Second Generation Foil Bearing: Effect of Shaft Temperature

“…References [8][9][10][11] detail measurements and procedures related to the experimental identification and prediction of GFB structural characteristics, namely, stiffness and damping. Accurate identification of a FB structure stiffness and mechanical energy dissipation provides reliable data to calibrate analytical tools modeling the intricate foil bearing underspring support structure.…”

Numerical and experimental investigation of bump foil mechanical behaviour

“…Similar experiments were performed by Rubio and San Andres [9,10]. These authors compared the experimental results to the ones obtained using a simplified mathematical model, in which the bump foil contribution was represented by simple elastic springs.…”