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Tribology and Lubrication Technology April 2012 : Page 33

When the oil is cool and clean but has high moisture influence, the moisture would dominate the decision (such as a paper mill). BASELInE SAMPLE InTERVAL Table Final interval Sample Interval Table 6 | 6. The The Final Sample Sum of Factors Default Interval, Days Final Sample Interval, Days 0.11 180 19.8 One might think that the lubricants industry has re-solved to a maximum viable interval but that isn’t the case. Lubricant suppliers have established sample intervals for many programs. This isn’t right or wrong, but it is inherently biased toward lubricant health and focus on the lowest value proposition for the entire exercise: lubricant replacement. In a clear-ly visible way, it is assigning the fox to guard the hen house. The base interval proposed for this method is 180 days, which is the longest reasonable interval that one would consider for oil sample collection, even if the objective is only to determine if it’s time to change oil or not. Addition-ally, the factor values are set to work with 180 days. If one chooses to follow the logic and create new, customized inter-vals that match more closely with company values, one could pick any default value that provides effective systematic re-sults. SAMPLE cALcULATIOn The final step is to multiply the summed weighted factors by the default sample interval. In this example, the weighted factors equal 0.11. As shown in Table 6, this value multiplied by the default value of 180 days produces a corrected sample interval for the specific machine of 19.8 days. The final sample interval calculation is: (WSF * Baseline) = Final Sample Frequency Interval, Days For whatever reason, the calculated interval is not ratio-nal (cost, harsh operating conditions justify shorter basis, machine is not accessible, etc.), adjustments are made to support site goals and/or the reality of site limitations. ness needs—not by consideration for the needs of the ma-chine and the customer’s enterprise. Since the best use for oil analysis is as a machine and sump condition assessment tool, an objective and systematic approach should be used to strongly influence sample inter-vals. There are four considerations that can be used to create interval discount factors for machines: (1.) rate of machine health progression from incipient failure to functional fail-ure, (2.) criticality level, (3.) environmental stress condi-tions and (4.) overall health characteristic. These four con-siderations should be weighted for machine type and plant priorities, with the weights totaling one. Once weighted, a factor is selected for each of the considerations. As the condi-tion defined by the factor progresses toward an extreme state, the associated discount number declines, producing a short-er interval. After selecting intervals and multiplying the selected in-terval by the weighting value, the net values are added to-gether. This final weighted value is multiplied by the default value, which is set at 180 days, to produce a realistic, reli-ability centered sample interval. SUMMARY Lubricant suppliers often set oil sample intervals. For better or worse, these intervals are driven by their company’s busi-WWW .S TLE. OR G TRIB OL OG Y & L UBRIC A TION TE CHNOL OG Y Mike Johnson, CLS, CMRP, MLTII, MLAIII, is the principal consultant for Advanced Machine Reliability Resources, in Franklin, Tenn. You can reach him at APRIL 2 012 • 33

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