**5. Conclusions**

For adoption in microturbines, the only conventional rolling element supports selected from manufacturers' catalogs (without including ad hoc solutions) compatible with the speeds at the hand are high-precision angular contact ball bearings, preferably with ceramic parts. Particularly, hybrid bearings allow for a perceivable increase in maximum speed, while all ceramic bearings can reach the high operative temperatures suitable for micro-GT units.

Air supports (hybrid aerostatic as well as aerodynamic) are the most suited oil-free solution for this application. They offer the best compromise between installation costs and performance for micro-GT systems. Aerodynamic bearings are preferred since they need no external supply of pressurized gas (e.g., tapped from the compressor) as they are self-acting. Particularly, foil bearings have good stability and compensate for shaft thermal expansion. Their main drawback is the short phase of dry friction during start up and stop, which limits their life.

Magnetic bearings are another promising oil-free solution. PMBs are suitable for speed and ease of miniaturization as well as independence from external energy input, but the high temperature limits the use of permanent magnets in microturbines since these could demagnetize. Consequently, electromagnets must be employed, although they consume a considerable amount of electrical energy. AMB remains the most suitable magnetic-driven solution because of high speed, temperature, and the control of machine dynamics.

The comprehensive comparison among the different reliable technologies carried out in the first part of this chapter proves that the design of an optimal support system might employ different type of bearings. Indeed, each of them have peculiar capabilities and limits, which make them suitable for particular tasks, e.g., carrying high load continuously or low load during frequent start/stops, working at high or low temperature, etc.

An innovative support system that relies on air as well as rolling element bearings and employs spline couplings has been proposed. The particular bearing arrangement devised for the new system is capable of taking advantage of the best characteristics of both bearing types and, simultaneously, of minimizing the effects of their flaws. Particularly, rolling element bearings behave much better than air bearings during start-stops but ensure limited duration under nominal load and speed of the most efficient micro-GT units. On the contrary, (air) foil bearings provide suitable life and load-carrying capabilities in nominal working conditions but cannot withstand a large number of start-stop cycles of the units. Therefore, the proposed system is designed to switch between the two types of bearings automatically when the unit ends the transient operation. In addition, it employs helical splines as both convenient coupling systems and actuators for the load partition between the two bearing types. Indeed, partitioning the turbine thrust is required in order to optimize the behavior of the support system.

The invention performs better than conventional support systems based on rolling element bearings, since it radically increases bearing life or load-carrying capacity as well as working temperature. In comparison with modern systems based on foil bearings, the invention is advantageous in that the number of start-stop cycles of the machine is not limited, solid lubrication is not required, the power of the starter can be reduced and retrofitting conventional machines based on rolling element bearings is possible. In any case, replacing shrink fittings with spline couplings can simplify maintenance and inspection of the impellers. Although the proposed support system is not oil-free, the concepts used in its development might be adopted in order to match other type of bearings. For instance, combining thrust foil bearings and radial magnetic bearings may yield an oil-free design with still higher performance.
