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START-STOP ANALYSIS OF HIGH-SPEED BEARINGS WITH A FLEXIBLE BUSHING

Wojciech Miaskowski

Abstract

The current demand for energy production in dissipated systems and the related need for miniaturization of power machines require the use of appropriate technical solutions. Increasingly, power generation microsystems are used to produce electricity that functions in hermetic circuits (e.g. ORC - Organic Rankine Cycle). Rotors of such type of machinery, in order to generate the expected amount of energy, must rotate at speeds up to 100,000 rpm. Rotational speeds of this order represent significant problems in the use of rolling bearings and the specific technological requirements associated with the implementation of the ORC circuit exclude the use of oil as a bearing agent, especially when bearing stability and ORC efficiency are prioritized. An alternative to this machine-type bearing is to use foil bearings operating in the presence of a turbine feeder, e.g. a low-boiling medium. Testing bearing system components for this type of machines has led to the need to build special test stands including the Start-Stop test stand. Start-stop tests for foil bearings are important because the primary reason for the wear of foil bearings is the friction between the bearing components, especially the top foil and the shaft neck, occurring during start-up and run-down of the machine and instantaneous rotor overloads. Start-stop testing allows the determination of the durability of foil bearings, but also provides the ability to determine basic operating parameters, such as bearing capacity and torque for starting and running conditions, which can determine the suitability of certain foil bearing design solutions and proposed material pairs for working elements of tested bearings. During start-up and run-down, the increased bearing resistance of the foil bearing is due to the formation of the bearing film (start-up) and its decay (run-down) during the relatively low rotational speed of the rotor pin, as well as the clip of the top foil on the shaft journal. During the first and last phase of the test, the bearing resistance torque results from the friction coefficient between the foil set faces and the journal, and the clamping force of the working foils on the shaft. An additional solution that contributes to decreasing of the foil bearing resistance and to reducing the wear of the foil set is the application of specially selected protective materials (e.g. ceramic, amorphous carbon, graphene, etc.) as a layer or coating covering the journal surface and the top foil. These solutions result in a reduction in the friction coefficient during the formation and disappearance of the carrier film and in an increase in the foil life.

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DOI resolver: doi.org/10.5593/sgem2017h/63/s24.009

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