Any company that has physical assets must maintain them over time, in order to maximize their return on those assets. Obviously, running all your assets to failure does not yield the highest return, but then neither does 100% preventive maintenance — that is, no assets run to fail. Comparing these two strategies in terms of cost, 100% preventive maintenance requires maintenance at regular intervals in order to prevent failure. This can be costly compared to simply replacing or repairing the asset upon failure. However, there are other factors that must be considered, such as the probability and impact of failure, the performance and reliability of the asset, and the age and condition of the asset.
Maintenance policy options
There are only three ways that maintenance can be triggered, regardless of asset type or its attributes. These correspond to the only three maintenance policies possible:
1. Use-based maintenance (UBM): This policy refers to maintenance triggered by time, meter, or event. For example, use-based maintenance on your car engine might be defined as changing the oil every three months, every 3,000 miles, or after every fourth tank fill-up. This corresponds to time, meter, or event driven UBM.
2. Fail-based maintenance (FBM): By running an asset to failure, you are adopting a fail-based maintenance policy for that asset. In the car example above, not changing the oil at all and running the engine to failure implies adopting an FBM policy.
3. Condition-based maintenance (CBM): The only other maintenance policy possible is condition-based maintenance — triggering maintenance when the condition of an asset exceeds an upper control limit, drops below a lower control limit, or trends in a given manner — for example, the slope is rapidly increasing. In addition to the actual maintenance work, a CBM policy necessitates regular condition inspections, from on-line real-time to manual inspections at regular intervals. Following the car example, choosing CBM over the other two policies would require regular inspections of the oil and sophisticated lubrication analysis to determine the level and characteristics of particulates. A cheaper alternative would be to visually inspect the oil to determine how black and dirty it is; however, this method is not as accurate in optimizing engine performance and lifecycle cost.
|David Berger, a Certified Management Consultant (C.M.C.) registered in Ontario, Canada, is a Principal of Western Management Consultants, based in the Toronto office. David has written more than 200 articles on a variety of topics such as maintenance management, operations management, information technology, e-commerce, organizational design, and strategy. In Plant Services magazine, he has written a monthly column on maintenance management in the United States, as well as three very extensive reviews of maintenance management systems available in North America. David has done extensive work in the areas of strategy, information technology and business process re-engineering. He can be reached at firstname.lastname@example.org.
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The three policies above replace the many words used to categorize maintenance work done. These include preventive, corrective, demand work, proactive, elective, predictive, daily/routine, scheduled, reactive, and many more. Ask three people in your company what these terms mean, including what triggers maintenance work, and you will undoubtedly get three different answers.
The most often used but least understood of these terms is “preventive maintenance.” If you ask your employees whether or not an inspection is preventive maintenance, most would say it is. This is false because an inspection is not maintenance at all. Nothing is repaired or replaced. It is a requirement of CBM in order to track when maintenance is triggered. If an inspection of the oil shows that replacement is required to prevent failure, then condition-based maintenance, not UBM, is performed. Use-based maintenance implies that the oil is simply replaced at a given interval, regardless of inspection results. Similarly, if upon inspection it is discovered that an asset has failed — for example, a light bulb is out — then the trigger for maintenance is failure, and it is classified as FBM.
The confusion lies in that companies mix the three policies on a given work order, thereby making it difficult to analyze asset history to determine the optimal maintenance policy. Keeping careful track of policies on a work order allows planners and asset owners to better understand for each policy, the relationship amongst probability of failure, impact of failure, asset performance, reliability, lifecycle cost, aging, and other factors.