MAINTENANCE INTERVAL SCHEDULING USING GRAPHICAL METHODS

摘要

Graphical methods using Total Time on Test (TTT) plots introduced by Barlow and Campo and latter advocated by Bergman and Klefso is handy to analyze maintenance/overhaul periods of repairable equipments. TTT plot can be used to monitor health of euipment in terms of constant failure rate/increaing or decreasing failure rate. It is presumed that these equipments are as good as new after each failure-repair process. The TTT plots can be drawn using Kaplan-Meier or Piecewise Exponential or Maximum Likelihood estimators. Simple graphical methods can be used to find out whether the equipment is improving or deteriorating. One such method is to draw a trend graph between cumulative time between failures (CTBFs) and the failure number. If the graph shows a trend of TBF's in the form of concave/convex upward, it indicates equipment is deteriorating/improving with time. This also indicates that TBF's are not independently and identically ditributed. Graphical methods like TTT plots can be used if the failure data satisfies i.i.d. asumption. The graphical representation provides visual display of the role of properties of failure rate function in determining optimal interval. In age replacement policy, it is assumed that a system is renewed by planned and unplanned maintenance either through replacements or repair carried out after failure.