Abstract:
Commercial turbochargers used on earth-moving equipment, were used as the turbo machinery for the
Pebble Bed Micro Model (PBMM) instead of custom-built turbo units. A premature thrust bearing failure
occurred on the High Pressure Turbo Unit (HPTU) of the PBMM during a test. Due to the closed
Brayton cycle operation of the PBMM, it might have been possible to exert larger thrust loads on the
thrust bearing of the HPTU than normally present in the turbo diesel engine application.
A detailed study was performed on theoretical thrust calculations as applied to turbo machinery. In the
literature study, five different thrust calculation methodologies were found. These are the methods of
Lobanoff & Ross, Stepanoff, Gulich, Japikse and Liebner. These methodologies were applied to make a
theoretical prediction of the thrust of the HPTU.
Experimental thrust measurement were performed on the HPTU, by implementing a metal foil strain
gauge thrust measurement system. Thrust load measurements on the HPTU and process parameters,
required for the calculation of the thrust, were recorded during a test run of the PBMM. The
experimentally measured thrust was compared with the theoretical thrust calculations. It was found that
the methodologies of Gulich, Lobanoff & Ross and Stepanoff correlated best with the experimental thrust
measurements.
Furthermore, calculations have shown that the thrust exerted on the thrust bearing of the HPTU in the
PBMM application, is approximately 1,4 times larger than in a normal turbo diesel engine application.
