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Lubricants and Lubrication
Published in Ahmed Abdelbary, Extreme Tribology, 2020
Like conventional gas bearings, foil bearings are hydrodynamic bearings that use the ambient gas as their working fluid. These bearings are composed of compliant surfaces and generally employ a gaseous lubricant. In both geometries a rotating member drags the viscous process fluid into a converging gap, raising the fluid pressure and providing a load carrying capacity, as shown in Fig. 5.18 (Dykas et al., 2008). Gas bearings do not function in a vacuum. Further, they rely on surface speed of the moving shaft, or in the case of axial thrust foil bearings, the runner surface, to generate the fluid film pressure. Since gases are thermally stable to high temperatures, one distinct advantage of foil bearings is their ability to operate from cryogenic to very high temperatures without experiencing a major change in bearing properties (DellaCorte, 2013). Figure 5.18 illustrates the configuration of a typical journal foil bearing and Fig. 5.19 depicts a thrust foil bearing that is used to control axial motion.
Sliding Bearings and Lubrication Mechanics
Published in Maurice L. Adams, Bearings, 2018
In stark contrast, operation of the magnetic bearing is anything but simple, involving position sensors, with A-to-D, microprocessor and D-to-A power amplifiers (Adams 2010); see Figure 3.1. But the magnetic bearing has quite predictable performance, for example, load capacity, dynamic stiffness, and damping coefficients. Whereas the much simpler configured foil bearing surely presents considerable challenges in predicting its operating performance characteristics, especially rotor dynamical stiffness and damping properties. The elastic deformation of the foils presents a significantly hardening nonlinearity with load, and it is the foil hardening nonlinear deformations that dominate both the static and dynamic-bearing properties. The beneficial damping inherent in the dynamic friction rubbing between the top and bumper foils is also a nonlinear mechanism. In consequence, use of foil bearings in a specific application requires significantly more development testing than other alternatives require. The main applications thus far for these foil bearings include turbochargers and micro-turbine engines for land-based electric power generation. The (1) compact oil-free nature and (2) high-temperature capability of the compliant surface foil gas bearing is significant.
Rotor–Stator Interaction
Published in Arthur W. Lees, Vibration Problems in Machines, 2020
Foil bearings have been known for a considerable time but interest has increased in recent years as part of general attraction toward oil free systems. Foil bearings are basically air bearings but with the addition of a flexible foil or membrane which takes the load at low speeds. As speed increases the film of air, the pressure field of which this develops viscous and inertia forces arising from the rotation supports load. The main areas of application are light, high-speed rotors. There has been increased interest in these bearings in recent years and Bonello (2019) has discussed the difficulties in evaluating their effective stiffness.
The dynamic pressure effect of gas microbearings under high rotational speed
Published in Engineering Applications of Computational Fluid Mechanics, 2022
Hydrogen fuel cell system, as a key component of the vehicle, determines the energy conversion efficiency and the vehicle performance. In this system, a high-speed air compressor is equipped to supply the fuel cell stack with high-pressured air for electrode reactions. At present, the rotational speed of this kind of centrifugal compressors is generally around 100,000 rpm (Chen et al., 2021; Shao & Zhang, 2019; Yz et al., 2020). As the air transferred into the fuel cell stack must be completely oil-free and pollution-free (Sun et al., 2021; Xu et al., 2020; Zhang & Shen, 2021), foil air bearings have become the mainstream choice for the current rotor support design (Daejong, 2021; Shi et al., 2022), for their low power losses at high speeds and robustness against acceleration and vibration. The related air circuit is relatively simple as well. Nevertheless, foil bearings have several disadvantages such as high startup torque, large overall size and weight (Metz et al., 2013). Additionally, foil bearings must reach a minimum rotational speed before establishing effective air pressure support. Therefore, under low speed, the foil needs to use its own elasticity to provide support for the rotor, which induces wear and tear between the bearing and the shaft during the startup and shutdown process of the compressor.
A Finite Element Model for Static Performance Analysis of Gas Foil Bearings Based on Frictional Contacts
Published in Tribology Transactions, 2021
Gas foil bearings (GFBs) have great potential in high-speed and advanced turbomachinery systems such as compressors and microturbines operating with a gas. Compared with rigid gas bearings, GFBs have better performance because of the compliant structure, which can generate additional damping and suppress vibration. Moreover, GFBs have the advantages of high running speed, wide temperature operation range, low frictional loss, and the omission of oil (1). There are many types of foil bearings, such as tension-type, leaf-type, and bump-type foil bearings. Among these types, bump-type foil bearings are the most widely used, because this kind of foil bearings exhibits better performance in terms of load capacity and stability. Many theoretical models have been proposed to predict the performance of GFBs and continuously promote their development. However, there are still many inherent challenges because the GFB system is multi-physical and highly nonlinear.