Session: 23-02: Gas Foil Bearings I

Submission Number: 178831

Lift Off Test in Gas Foil Bearings: a Comparison Between Friction Torque and Electrical Contact Resistance Measurements 

Gas foil bearings (GFBs) are increasingly used in small, high-speed turbomachinery. These oil-free, aerodynamic bearings feature a compliant foil inner structure that compensates manufacturing and alignment errors, while providing the bearing with additional damping due to the friction between the foils. The functionality of GFBs is verified through a lift-off test, which involves measuring the friction torque (FT) applied by the rotor and/or gas film to a “floating” GFB during transient runs. The lift-off is then identified by comparing the results with the Stribeck curve. However, the FT method suffers from several limitations that make it inaccurate and ineffective at capturing transitions between lubrication regimes. The definition of lift-off itself remains inconsistent across the literature, and discrepancies between the assumptions of FT measurements and the Stribeck curve are often overlooked. While the Stribeck curve represents steady-state conditions, FT tests are inherently transient, with parameters such as temperature not reaching equilibrium during the tests. Another factor normally neglected is foil deformation, which makes lubrication in GFBs an elasto-aerodynamic phenomenon rather than a purely aerodynamic one, differing from the assumptions of the Stribeck curve again. Electrical contact resistance (ECR) measurements have long been used to investigate hydrodynamic lubrication regimes in oil-lubricated bearings as an alternative method. In this technique, the electrical resistance between the bearing and shaft, which is directly related to the amount of surface contact, is measured to detect lubrication regimes and hence the lift-off state. In this study, the ECR method is applied for the first time to identify lift-off in GFBs, with FT recorded simultaneously. The comparison between the two demonstrates the shortcomings of FT measurement in identifying the lift-off point accurately.

 

Keywords: gas foil bearing, electrical contact resistance, friction torque, lift-off

Presenting Author: Majid Ahmadzadeh Technische Universität Berlin

Presenting Author Biography: Majid Ahmadzadeh completed his master’s degree in Mechanical Engineering at TU Berlin in 2021. Since 2022, he has been working as a research assistant at the Department of Engineering Design and Product Reliability at TU Berlin. His research interests include rotordynamics and metrology, with a focus on the instrumentation and condition monitoring of gas foil bearings.

Authors:

Majid Ahmadzadeh Technische Universität Berlin
Martin Kliemank Technische Universität Berlin
Lukas Wedenig Technische Universität Berlin
Clemens Gühmann Technische Universität Berlin
Robert Liebich Technische Universität Berlin