# Gas Foil Bearing Technology Enhanced with Smart Materials
## 2 Demands for Improving Characteristics of Gas Foil Bearings  
Although GFBs exhibit many advantageous properties (as referenced in Section 1), it should also be highlighted that there exist some inconveniences that have to be addressed to assure their proper operation [4]. First of all, load carrying capacity in GFBs is much smaller compared to the liquid lubricated bearings. This important limitation may be partially reduced by increasing the GFBs' rotational speed. Anyway, this inherent feature of GFBs originates from low viscosity of a gaseous medium used. Effectively, only lightly-loaded components of rotating machines may be supported with the mentioned type of journal bearings. Next, even though the GFBs' construction is simple, there is a need for high manufacturing tolerance to keep the correct distance between the shaft's journal and the top foil, i.e., the bearings' clearance. A small change regarding this clearance may lead to a significant reduction in the generated pressure profile and, hence, loss of nominal load carrying capacity, again due to low viscosity of the bearing's lubricant. In reference to the demanding manufacturing quality, there is also high misalignment sensitivity found for GFBs. In fact, the higher precision assured for the bearing's clearance, the more demanding orientation and position alignments required for the cooperating GFBs. Alignment errors influence the bearings' clearances and the capability of normal operation. Another issue is the maintenance of thermal stability of GFBs [11; 12]. As discussed in Section 1, GFBs may operate continuously at high ambient temperatures. However, excessive temperature gradients are very dangerous for this type of bearing. Below, the authors briefly explain the physical phenomena related to the discussed GFBs' property. During normal operation, i.e., under nominal loads, the developed air film allows for both hydrodynamic pressure generation and heat flow in a bearing. The latter assures the desired level of temperature homogenization within the bearing's body. Nevertheless, the maximum value of the temperature gradient is found in the region of a mechanical load transfer, due to the raised shear forces in the lubricant, existing in the mentioned localization. Expectedly, in the case of overloading and possibly too low rotational speed, the bearing's clearance may become extremely thin which results in the raised temperature gradient. Next, if none of additional, i.e., externally forced, heat flow mechanism is activated, the excessive gradients may rapidly lead to uneven thermal expansion of the bearing's structural components. Thermally induced modification of the specialized foils' geometry, e.g., wrapping their edges, may result in blocking the channels for lubricant flow or even rotor stuck in the GFBs' pushing. The described issue is known as thermal stability loss (or the phenomenon of thermal instability) in GFBs. It is worth noting that thermal instability may also be observed when either the average temperature of the bearing or ambient temperature are low [13; 14; 15]. Consequently, dedicated thermal management may be considered an effective solution to decrease undesired gradients of the temperature. Inreference, a comprehensive analytical and numerical study on the influence of temperature on GFBs' performance can be found in [5]. Predicting the static and dynamic characteristics of GFBs is also a major challenge. Due to the fact that these bearings are very complex mechanical systems, where flow and structural phenomena and also fluid-structure interactions occur, the theoretical modeling of GFBs is very difficult. Reliable and experimentally verified computational models of such bearings make it possible to predict their characteristics for a wide range of operating parameters. Dynamic stiffness and damping coefficients are very important in this case.