<div class="htmlview paragraph">This paper presents the results of measurements to determine the effect of contaminant particle size on wear in lubricated systems, intended to increase understanding of the needs of lube-oil filtration systems for engines and transmissions. The results indicate that component wear rate is independent of contamination size when particles down to at least 3 µm are present. By-pass filtration theory is used to explain why current filtration systems are unable to remove contamination this small and that an alternative is needed if oil and component lives are to be extended beyond current levels. Test results are included which confirm this conclusion and show that because its performance does not deteriorate during its life, centrifugal filtration is a viable alternative.</div>