Comparative Analysis of Bearings for Micro-GT: An Innovative Arrangement

Microgas turbines are a widespread technology in cogenerative and propulsion applications. Bearings are a key factor in their design and development. The aim of the present research work is the development of the support system for a typical microturbine intended for power generation. To this goal, the present chapter defines the typical requirements of the machine and, afterward, describes the different technologies available to develop the support system of a reliable microturbine. Conventional (rolling element and oil-film) supports and cutting-edge (magnetic, aerodynamic, and aerostatic) bearings are reviewed. Particularly, their suitability to the operating conditions is compared by means of a literature review and elaboration of the relevant data. By analyzing all this information, a new concept for the design of a micro-GT support system is devised. Instead of using a single type of bearing as usual, the new system includes different types in order to take advantage of the best characteristics of each one and, simultaneously, to minimize the effects of the relevant flaws. The innovative support system requires a suitable bearing arrangement, which is compared with the conventional ones. The conceptual design of the innovation is completed by a discussion of its advantages, drawbacks, and prospective improvements

Static, dynamic and levitation characteristics of squeeze film air journal bearing: Designing, modelling, simulation and fluid solid interaction

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Bearings today need to be able to run at very high speed, providing high positional accuracy for the structure that it supports, and requiring very little or no maintenance. For this to happen, bearings must have tight tolerances and very low or zero friction during operation. This pushes many traditional contact-type bearings to their limits as they often fail due to friction, generating heat and causing wear. By comparison, existing non-contact bearings fare better because of their very low or zero friction. But some have their own problem too. For example, the fact that aerostatic bearings require an air supply means having to use a separate air compressor and connecting hoses. This makes the installation bulky. Aerodynamic and hydrodynamic bearings cannot support loads at zero speed. Both hydrodynamic and hydrostatic bearings may cause contamination to the work-pieces and the work environment because of the use of lubricating fluid. A potential solution to the above-mentioned problems is the new squeeze film air bearing. It works on the rapid squeeze action of an air film to produce separation between two metal surfaces. This has the benefit of being compact with a very simple configuration because it does not require an external pressurized air supply, can support loads at zero speed and is free of contamination. For this research, two squeeze film air journal bearings, made from material of Al 2024 – T3 and Cu - C101 with the same geometry, were designed. The bearing is in the shape of a round tube with three fins on the outer surface and the journal, a round rod. When excited at a certain normal mode, the bearing shell flexes with a desirable modal shape for the squeeze film action. The various modes of vibration of Al bearing were obtained from a finite-element model implemented in ANSYS. Two Modes, the 13th and 23rd, at the respective frequencies of 16.320 kHz and 25.322 kHz, were identified for further investigation by experiments with respect to the squeeze film thickness and its load-carrying capacity. For Cu bearing, the two Modes are also 13th and 23rd at the respective frequencies of 12.184 kHz and 18.459 kHz. In order to produce dynamic deformation of the bearings at their modes, a single layer piezoelectric actuator was used as a driver. The maximum stroke length and the maximum blocking force of the single layer piezoelectric actuator were determined using manual calculation and ANSYS simulation. In the coupled-field analysis, the single layer piezoelectric actuator was mounted on the outside surface of the bearing shell and loaded with an AC and a DC voltage in order to produce the static and dynamic deformation. For the static analysis, the maximum deformation of Al bearing shell is 0.124 μm when the actuators are driven at the DC of 75 V. For the dynamic analysis, the actuators are driven at three levels of AC, namely 55, 65 and 75V with a constant DC offset of 75V and the driving frequency coincided with the modal frequency of the bearing. The maximum dynamic deformation of Al bearing shell is 3.22μm at Mode 13 and 2.08μm at Mode 23 when the actuators were driven at the AC of 75 V and the DC of 75 V. Similarly, the FEA simulation was used for analyzing Cu bearing. Furthermore, the dynamic deformation of both Al and Cu bearing at Mode 13 and 23 are validated by experiments. This research developed two theoretical models that explain the existence of a net pressure in a squeeze film for the levitation. The first model uses the ideal gas law as first approximation whilst the second uses the CFX simulation to provide a more exact explanation. In terms of the load-carrying capacity, Mode 13 was identified to be better than Mode 23 for both bearings. However, at Mode 13, Al bearing has a higher load-carrying capacity than Cu bearing. This is due to Al bearing having a higher modal frequency and amplitude. Finally, the coupled-field analysis for fluid solid interaction (FSI) was studied at both Mode 13 and 23 for Al bearing. The findings are that: a) the fluid force in the squeeze film can affect the dynamic deformation of the bearing shell, especially at high oscillation frequency, more at Mode 13 than at Mode 23 due to the relatively high pressure end-leakage in the latter; b) the dynamic deformation of the bearing shell increases with the gap clearance in a logarithmic manner at Mode 13; and c) the micron levels of gap clearance provide a damping effect on the dynamic deformation of the bearing shell at Mode 13 and at Mode 23, though much less dominant

Investigation of non-contact bearing systems based on ultrasonic levitation

Su ZhaoPaderborn, Univ., Diss., 201

Model Identification, Updating, and Validation of an Active Magnetic Bearing High-Speed Machining Spindle for Precision Machining Operation

High-Speed Machining (HSM) spindles equipped with Active Magnetic Bearings (AMBs) are envisioned to be capable of autonomous self-identification and performance self-optimization for stable high-speed and high quality machining operation. High-speed machining requires carefully selected parameters for reliable and optimal machining performance. For this reason, the accuracy of the spindle model in terms of physical and dynamic properties is essential to substantiate confidence in its predictive aptitude for subsequent analyses.This dissertation addresses system identification, open-loop model development and updating, and closed-loop model validation. System identification was performed in situ utilizing the existing AMB hardware. A simplified, nominal open-loop rotor model was developed based on available geometrical and material information. The nominal rotor model demonstrated poor correlation when compared with open-loop system identification data. Since considerable model error was realized, the nominal rotor model was corrected by employing optimization methodology to minimize the error of resonance and antiresonance frequencies between the modeled and experimental data.Validity of the updated open-loop model was demonstrated through successful implementation of a MIMO u-controller. Since the u-controller is generated based on the spindle model, robust levitation of the real machining spindle is achieved only when the model is of high fidelity. Spindle performance characterization was carried out at the tool location through evaluations of the dynamic stiffness as well as orbits at various rotational speeds. Updated model simulations exhibited high fidelity correspondence to experimental data confirming the predictive aptitude of the updated model. Further, a case study is presented which illustrates the improved performance of the u-controller when designed with lower uncertainty of the model\u27s accurac