# Gas Foil Bearing Technology Enhanced with Smart Materials
## 5 Applications of Piezoelectric Transducers to Gas Foil Bearings
The PZTs are mounted directly under the bearing's foils. Hence, both local and global change on the air film thickness (i.e., shape modification for the bearings sleeve) is feasible. The authors present the results of numerical studies conducted for various operational scenarios changing the nominal clearance, preload and voltage which supplies PZTs. The proposed solution enhances the dynamic response of the bearing in terms of vibrations that may appear for natural frequencies. The conducted numerical simulations provide shaft's journal eccentricities and trajectories, stiffness and damping coefficients as well as transient responses for various rotational speeds. As confirmed, the activation of PZTs advantageously makes it possible to safely pass across the critical rotational speeds. The studies [26; 28; 29] report the application of circumferentially oriented PZTs built-in into the body of the GFBs' bushing. As presented in the cited works, it is feasible to adjust mechanical preloads in the bearing making use of PZTs and mechanical amplifiers (lever amplifiers) constructed with flexures. The desired radial displacement of the GFBs' foils is achieved after voltage activation of the PZTs. The lever amplifiers allow for gaining the effect of the electrically induced strain of the PZTs. As proven based on the results of experimental and simulation analyses, the proposed solution exhibit the following features: controllable clearance, stiffness and the shaft's journal eccentricity, which results in higher operational stability. The earliest paper [26] presents the model of the above-discussed active bearing. The model takes into account both the structural passive parts, i.e., the specialized foils, and the active components--PZTs. Linked springs and rigid links have been used to approximate the properties of the GFBs' foils. Drag torque, resultant stiffness and damping coefficients and eccentricity are found with the elaborated model subjected to various supply voltages. The work [28] is a continuation of the above referenced numerical study. The authors investigate the properties of a nonlinear model of an actively controlled GFB. The nonlinear dynamic responses of the bearing are analyzed making use of the shaft's journal trajectories with relevant Poincare maps and fast Fourier transform. Additionally, the minimum thickness of the air film and the amount of dissipated energy for a single cycle of the shaft's orbit are identified. The elaborated model has been successfully verified using experimental results. Further results of experimental investigations are presented in the complementary paper [29]. The work describes the behavior of the tested GFBs' installation in the presence of real-time controllable mechanical preloads. Moreover, the influence of the shaft unbalance on the bearing's response is also discussed. The authors have also identified the critical rotational speeds for various voltages. The study [33] reports a hybrid bearing that integrates a common ball bearing and an internally mounted GFB. In the proposed solution, the GFBs' clearance is actively adjustable via electromagnetic coils installed in the bearing's housing. The inner ring of the ball bearing is held using PZTs. The authors show the capability of mechanical vibration reduction in the shaft's journal during the bearing's operation. A dedicated control system may effectively change the resultant elasto-damping characteristics of the GFB component that enhances the hybrid bearing's performance.