Graphical methods for reliability of repairable equipment and maintenance planning

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 equipment. Simple statistical and graphical techniques are used to analyze failure data which exhibit independently and identically distributed (IID) pattern. Weibull or other types of renewal process models are used to model failure times. TTT plots can be used to monitor health of equipment in terms of constant failure rate/increasing or decreasing failure rate. It is presumed that these equipment 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. However, data, which does not satisfy IID assumption and where trend is observed. Nonhomogenous poisson process models, can be used, which represent repairable equipment with minimal repair. Maintenance planning with optimal overhaul periods based on minimum cost per unit time can be obtained graphically for repairable equipments experiencing increasing failure rate (IFR). Further, repairable equipments, whose performance is effected by concurrent variables, can be modelled by proportional hazard models. Graphical methods can be used to arrive at maintenance intervals. This paper discusses some of the graphical tools mentioned above to analyze failure data and plan maintenance intervals for material handling equipments operating in the mining industry