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

Fabrizio Stefani, Andrea Perrone, Luca Ratto and Ramon Francesconi

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67147

#### **Abstract**

determined. General approach for the calculation of bearing overall dimensions is consid‐ ered. In Chapter 5, motor current signature analysis based on squared envelope spectrum is applied in order to identify and to estimate the severity of outer race bearing faults in induc‐ tion machine. Bearing fault characteristic components are extracted combining summation of phase currents, pre-whitening, spectral kurtosis and squared envelope spectrum analysis. I have endeavoured to maintain the material presented in an up-to-date and useful format. I hope that the readers especially young researchers and engineers in this field will find this book as useful and informative. I feel amazing pleasure to edit this book. I would like to express my sincere gratitude to all authors for their outstanding chapters. I also wish to ac‐ knowledge the InTech editorial staff, in particular, Mr. Edi Lipovic, Publishing Process Man‐ ager, for indispensable technical assistance in book preparation and publishing. And of course, I would like to thank my parents, wife Veera and loving son Fagun for the support

**Prof. (Dr.) Pranav H. Darji**

C. U. Shah University

Director, P. G. Studies and Research

Wadhwan City, Gujarat (India)

Professor and Head, Department of Mechanical Engineering

C. U. Shah College of Engineering and Technology

Research, Development and Innovation Centre

they always gave to me—thanks for being close to me during all my life.

VIII Preface

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.

**Keywords:** microturbine, gas-turbine, bearing arrangement, bearing performance, foil bearings

#### **1. Introduction**

Microturbine technology owes its origins to the military and aerospace industry, where the need of compact and high power density engines justifies significant production and development costs. Later, micro-gas turbine (micro-GT) units have been used in small-scale power

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generation as well as cogeneration, and they are involved in the air compression as well as conditioning market. Recently, manufacturers are addressing their efforts to new market areas, for example, in powering hybrid electric vehicles as well as autonomous robots in the case of small-size machines.

In the following, such microturbines are intended as autonomous power generators. Despite their name, excluding portable devices and MEMS, typical shaft diameters of commercial microturbines are roughly 10 mm, and their electric power is in the order of 100 kW.

In the design of a microturbine, the bearing choice is not a trivial issue due to the high rotation speed and working temperature. In these operating conditions, the "classical" engine design criteria for choosing the most adequate bearing type (rolling or sliding bearings), which are based on dimensions and/or nominal power rate, cannot be adopted. According to such traditional design criteria, the choice for machines of small and large dimensions is unavoidable, i.e., rolling and sliding bearings are employed, respectively. For intermediate power machines, on the contrary, specific choices have to be carried out: on the one hand, rolling bearings have smaller overall dimensions and purchasing costs, and on the other hand, sliding bearings are more reliable.

Therefore, for microturbines, a comparative analysis of the available support systems, particularly focused on studying the influence of operating conditions (speed, temperature, and loads), is required. Accordingly, the present chapter provides a detailed comparison of bearing technologies by means of literature review and analysis of published data. Cutting-edge (e.g., magnetic, air, and ceramic bearings) and well-established solutions (e.g., steel rolling element and oil-lubricated slide bearings) are both considered in order to provide a large perspective.

On the basis of such a comparative study, an innovative design of a support system for micro-GTs capable of overcoming the limits of modern units is proposed. It employs different types of bearings and requires a proper design of their arrangement and coupling in order to take the maximum advantage of the peculiarity of each bearing. After a brief description of the existing bearing arrangements, the present chapter deals with the conceptual design of the new support system.

In synthesis, the present chapter defines the bearing requirements on the basis of design specifications of a typical micro-GT unit. Afterward, it studies the suitability of different bearing types for the operating condition requirements by means of a literature review. Finally, the innovative support system is proposed and compared with the current technical solutions.
