A Numerical Analysis of the Nonlinear Dynamics of Shimmed Conical Gas Foil Bearings

Abstract

Conical gas foil bearings (CGFBs) have the potential to halve the number of necessary bearings in a conventional rotor system supported by gas foil bearings. Transient simulations of gas foil journal bearings and gas foil thrust bearings have proven their necessity to accurately predict the safe operating range of such bearings. This work presents the first transient model able to simulate the three-dimensional dynamics of CGFBs. The static behaviour of a single CGFB with a uniform bearing clearance is compared to a CGFB modified by thin metal shims and the resulting advantages of shimmed CGFBs are discussed. An investigation of the linear stability behaviour shows that the axial load of the bearing determines the stability of the equilibrium position. Furthermore, three transient simulations demonstrate the capability of the presented model to describe the nonlinear dynamics of a shimmed CGFB such as the occurrence of stable limit cycles and self-excited sub- and super-synchronous vibrations with and without a rotor unbalance. Additionally, waterfall diagrams are used to investigate the frequency response for different rotational speeds. The novel findings of this work are the importance of a non-uniform bearing clearance for the functionality of a CGFB and the identification of the axial force as a critical factor in maintaining bearing stability. These findings are specific to CGFBs and have not been discussed or mentioned in previous works.

Calculation of Dynamic Coefficients of Air Foil Journal Bearings Using Time-Domain Identification

“…As the dynamic viscosity coefficient of air is low, the air foil bearing provides limited damping under high-speed conditions and may suffer from hydrodynamic instability; thus, dynamic design and analysis are necessary for the safe operation of a rotor. Obtaining the dynamic characteristics of a bearing is a prerequisite for rotor dynamic design [5,6]. The dynamic coefficient of a bearing is typically used to describe the dynamic characteristics.…”

Special Issue on Rotor Dynamics: Theoretical Analysis, Computer and Experimental Modelling, Measurements

“…The conducted research showed that the operating characteristics are highly dependent not only on the used material and bearings' geometry but also on the surface roughness of the mating parts. Sarrazin and Liebich [3] concentrated their study on conical foil bearings. Their work's novel findings show that the investigated bearings' proper functionality requires a non-uniform bearing clearance and identification of the critical axial force for operation stability.…”