Preventive Maintenance Optimization
by Christer Idhammar
For decades, many “experts” have used the graph in Figure 1 to discuss the optimum level of maintenance. This Figure is based on an old-fashioned, yet widespread approach that bases preventive maintenance on Fixed Time Maintenance (FTM) replacements and overhauls of components. This approach is seldom justifiable because only 15% to 20% of all components fail after a predictable time.
As the graph shows, the more this type of maintenance is performed, the larger the cost for preventive maintenance becomes. Concurrently, the cost for corrective maintenance is supposed to go down. The cost for production losses is believed to go down to a point where you need to shut down equipment more frequently for preventive maintenance, and, as a result, the cost of lost production starts increasing.
The top total curve in Figure 1 is the sum of the other curves and shows what the optimum level of maintenance should be.
THE RIGHT APPROACH. A modern and cost-effective approach to preventive maintenance shows that there is no maintenance cost optimum. Instead, maintenance costs will decrease at the same time as costs for production losses also decrease. This approach can be summarized as follows.
No preventive maintenance action is performed unless proven to be less costly then the failure. A simple consequence of failure analysis (CFA) is made to justify preventive maintenance activities.
Preventive maintenance activities are primarily condition-based. The condition of a component, measured when the equipment is in operation, governs planned and scheduled corrective maintenance. It is acceptable to operate a component to breakdown when it is the most cost-effective maintenance procedure. A standard corrective maintenance procedure should be developed and documented.
Define the need for corrective maintenance early on (Condition Monitoring) as a part of preventive maintenance. Correcting the problem is defined as planned and scheduled maintenance.
Practicing this maintenance philosophy will transform the earlier graph to that of Figure 2. Here, the cost for preventive maintenance is very low, especially if operators are trained and motivated to do some essential care and inspections. A plant might end up having less than 5% of the traditional work force allocated for preventive maintenance. As with Figure 1, the top total curve in Figure 2 is the sum of the other curves and shows what the optimum level of maintenance should be.
With this improved maintenance philosophy, the level of planned and scheduled corrective maintenance will increase to over 80% and total maintenance volume and costs will go down 20% to 30%.
More significantly, reliability will improve and production throughput will be faster. This results in lower costs for lost production. And, because preventive maintenance activities are cost-justified based on failure-developing periods and failure distribution in time, total costs are continuously decreasing.