4. THRUST BEARING SERVICE LIFE AND LUBRICATING OIL SYSTEM
The rise in compressor pressure ratio can result in the journal bearings operating at high rotational speeds and in the thrust bearings carrying increased loads. Consequently the thrust bearing service life should be expected to vary much as the compressor impeller service life, warranting the provision for an appropriate lubricating oil system as well as careful maintenance and servicing.
(1) Foreign Particles Contained in Lube Oil and Bearing Wear
Since MET turbochargers or turbochargers of other makes as well, share the lubricating oil system with diesel engines they serve, it is invariably the case that lube oil supplied to the turbochargers contains foreign particles carried over from the engine lubrication. The foreign particles, promoting the turbocharger bearing wear, deserve every effort to reduce them as far as is practicable in order to extend the thrust bearing service life in particular.
When analyzing lube oil for impurities, the generally accepted practice of measuring the quantity of pentane insolubles falls short of accurately evaluating the effect of the foreign particles contained in lube oil on the bearing wear. Both the size and quantity of foreign matters contained in lube oil require to be determined for the prediction of service lives of turbocharger bearings, especially the service life of each individual thrust bearing, lubricated by the common lubricating system that serves the diesel engine as well.
By way of example, the results of lube oil analysis for foreign contents by size and quantity conducted following the NAS classification method are shown in Fig.6. It can be said from the past experience that when the quantity of foreign matters exceeds the Grade 12 of NAS classification, the thrust bearing service life goes down.
(2) Lubricating Oil System
Fig.7 shows examples of lubricating oil system variations so far employed for typical MET series of turbochargers.
Indicated to the left is the lubricating oil system employed for the MET-S model, in which a 250 mesh lube oil filter is installed before the turbocharger. In this system, the 250 mesh lube oil filter, being smaller in mesh size than the engine lube oil filter, catches all the foreign matters carried over from the engine. The experience shows that in such an instance, the turbocharger lube oil filter differential pressure tends to rise and that the foreign matters caught in the lube oil filter are forced through the filter.
Shown to the right of Fig.7 represents the lubricating oil system morest prevalently employed at present. The engine lube oil filter is 250 mesh, the same size that is required for the turbocharger lubrication, and adequate in capacity to handle a large quantity of lube oil required for the entire engine lubrication. This lubricating oil system, being capable of catching foreign matters efficiently and equipped with an oil purifier, dispenses with the lube oil filter before the turbocharger.
Another lubricating oil system also shown to the right of Fig.7 as improved type is the one in which a 500 mesh continuous back-
