May - Library

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Downtime Priorities, Jack-Knife Diagrams, Business Cycle
Figure 8. Combined Reliability, Availability and
Maintainability Limits
Figure 9: Jack-knife diagram for commodity price cycle peaks
Figure 10: Jack-knife diagram for commodity price cycle troughs
Experience with analysing larger sets of downtime data has shown
that the priority list simply grows too large. To this extent, it is
desirable to focus on only those chronic failures that have highest
availability impact.
6. Root Cause Analysis and Remedial
Action
Once a prioritised list of failure codes has been identified,
hypotheses can be made about the possible cause (or causes) of each
p roblem. Experienced maintenance and operating personnel are
indispensable to this process, since familiarity with the machine, the
operating environment and with maintenance and operating practices
is required.
The root causes of equipment downtime are associated with the
following broad categories; equipment design, purchase, storage,
installation or start-up, operation or maintenance (Moore, 2002). Each
of these categories can be further subdivided; for example, ro o t
causes associated with maintenance include: planning and
scheduling problems; re s o u rce availability problems (labour, tools,
documentation, shop space); inadequate inspection or PM practices;
and poor work quality.
C h ronic repairs are often associated with design pro b l e m s
(material quality defects or design inadequacies), inappro p r i a t e
operator practices or poor quality control in upstream processes. Tw o
good examples of chronic repairs are provided by the data:
Figure 11: Trends in unplanned failures for
BE 495-B Cable Shovel
Motor over- t e m p e r a t u re alarms (failure code 11) can result from poor
blast fragmentation, shovel abuse or high ambient temperatures. In the
former two cases, corrective action should be directed at mine
operations.
Outages to the shovel lighting system (failure code 9) typically
result from wiring damage due to structural vibration or poor filament
re l i a b i l i t y. Redesign of the wiring harness may be one way of tackling
this problem.
Following the identification of the factors contributing to the root
causes to each failure code, a set of corrective actions should be
f o rmulated to eliminate or mitigate the factors causing unplanned
downtime. Some maintenance actions may necessitate investment on
the part of the mine. An estimation of the expected reduction in
downtime allows the Maintenance Department to undertake a
cost/benefit evaluation of implementing the maintenance action plan.
If the cost savings are projected over say, a 3-year period, an NPV
can be calculated for the maintenance project. The advantage of this
a p p roach is that it permits executive management to evaluate
maintenance projects alongside competing project altern a t i v e s .
Maintenance need no longer be perceived as a costly overhead, but
as a strategic tool to maximise asset utilisation.
7. Trend Plots
A further benefit of logarithmic scatterplots is that they provide a
useful means of visualising trends in maintenance performance. For
example, Figure 11 shows the evolution of four failure codes from a
BE 495-B cable shovel working at an open pit copper mine in Chile.
Unplanned failures were analysed for a period of three years, 1997 to
1999 inclusive. The threshold limits used in the graph were calculated
relative to the total unplanned failure data set for the three year period.