Maintenance strategy is often explained as a decision rule that has to determine the order of necessary actions which should be performed in order to keep the system which we are using in the company working properly. It provides information about scheduling and it can be assigned to as many tasks (for example project manager's tasks) as needed. Maintenance strategies are meant to improve quality and productivity while reducing costs of operations.
Common maintenance strategies
Maintenance strategies fall into one of three types: corrective (reactive), predictive and preventive.
Most common maintenance strategies are: run to breakdown, time-based maintenance and condition based maintenance.
- Run to breakdown maintenance (RTB, also called run to failure or failure ending maintenance) - allowing operations to run until they fail. Maintenance work is only performed after failure has taken place. This strategy is often implemented when breakdown has minor impact and/or is easy to repair.
- Time-based Maintenance (TBM) - scheduling work to reduce failure chances. This strategy is useful when operation has to run continuously and error-prone in given periods of time and maintenance may be undertaken in idle time. Other important factor in choosing PM strategy may be low maintenance cost compared to failure impact.
- Condition-based maintenance (CBM) - monitoring characteristics of operation to plan maintenance when failure is imminent. CBM can be used in operations where breakdown can be predicted and it's cost is high. The latter might be true in cases where failure would mean disruption to critical operation or when maintenance itself is expensive.
System or operation might use few maintenance strategies at the same time. A car mixes all three approaches. Replacing light bulbs usually happens when they fail (RTB), engine oil should be changed regularly (PM) while many components might have an indicator when they start to exhibit unusual behavior (CBM).
Choosing maintenance strategy
When choosing maintenance strategies, different factors must be considered. All maintenance strategies must have defined parameters to allow continuity of operations and risk management. Economical factors in choosing maintenance planning are crucial. This should especially include costs of both repair and downtime in case of failure. Evaluating consequences (economical, environmental, safety, etc.) of failure is useful in answering how important it is to prevent failure from happening. Knowing probability of failure is useful to adjust maintenance schedule. Time-related parameters help to define maintenance strategy. Time horizon might be distance in future to maintenance or intervals between consecutive tasks. Shift factor is allowed deviation from time horizon.
One of the most important tools in maintenance planning is risk-based maintenance (RBM). It is methodology which tries to minimize effort and cost of maintenance while choosing effective strategy. Risk is calculated as a product of probability and consequence of failure. Any given area of operation is rated by its risk factor. Then all values are juxtaposed and priotizations might be assessed. This tool is very useful for preventive maintenance planning. There are many variants of RBM. Risk-based methodology, while simple in concept, requires in-depth risk analysis and different techniques might used to perform it. It must be noted that risk values are always subjective and prioritizations based on this approach are debatable.
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Author: Karolina Próchniak