The Road to Maintenance and Reliability: What You Need to Know [Part 2]
by mark west, director of reliability
6- A Primer for Asset Criticality Ranking
Asset criticality ranking helps us set priorities for both maintenance and reliability. This means that performing a quality criticality ranking is imperative to effective and efficient operations. The criticality I’m referencing is called “business criticality”; it is different from safety criticality and is not interchangeable. Business criticality takes multiple factors such as failure effects on operations, safety, environment, quality and more to produce a numerical score that allows direct comparison to other assets. This scoring reflects the relative severity of impact for reasonable worst-case failures—not how likely it is.
The numbers derived are important for calculations and prioritization of corrective actions. However, for improvement purposes, we must also consider how likely the failure is to occur to determine where to invest resources in preventing failure. By looking at the level of currently invested resources, we can temper the relative ranking for identifying opportunities to prioritize work as well as improve the level of effort put into maintenance and reliability activities. Can your organization point to a procedure that demonstrates why an asset has the criticality ranking assigned?
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7- Maintenance Strategy Determination
Original Equipment Manufacturers (OEM) are slowly changing with the times; however, we are not all dealing with new equipment or the OEMs that are changing. I bring this up as support for why simply following an OEM’s recommendations for maintenance may not be optimal.
Imagine that you are the OEM. You have a piece of equipment you are selling that contains a greased bearing. You know the bearing won’t last forever, so you need to tell the customer when to replace it. You sell thousands of these pieces of equipment, so you don’t create customized maintenance plans based on the customer’s abilities or the operating context of the equipment; you simply write generic maintenance guidelines. You know the L10 life of the bearing is 2000 hours of operation (90% of the bearings will make it at least this long) and the mean time between failures (MTBF) of the bearing is around 5800 hours. When do you tell the customer to replace the bearing?
We can assume the right answer is, “When it needs to be replaced based on vibration measurement,” but you as the OEM cannot count on the customer having this capability. The real answer is probably going to be, “I set the interval at a point that we will achieve the fewest complaints.” If you set it too low, people won’t like changing bearings all the time and seeing that they are fine. If you set it too high, there will be too many failures. If you set it somewhat high, the failures that occur will be accepted by those who count on OEM maintenance because “these things just happen.” Advanced customers will develop their own predictive maintenance plans and replace the bearing only when it needs replacing. How much OEM-based planned maintenance (PMS) are you doing that are not value-added?
8- How an Insurance Mentality Leads to Overstocking
The first question I must ask is, “Why do you keep spare parts?” The typical answer is commonly, “To fix stuff when it breaks.” If this is your answer, you may be missing some key concepts of maintenance resulting in reacting to events instead of anticipating them. When we are in reactive mode, the only way to store spare parts is to buy at least one of everything that might break. This is called an “insurance mentality.” You don’t want to get caught not having the part when the equipment breaks! This typically occurs when the reasons for equipment failing are not understood and adequate mitigating activities are not in place to prevent or detect the failure starting to occur.
On the surface, this may not appear to be a bad thing. Many organizations have operated this way for so long that it is normal to them. It is even possible that the majority of failures are so minor and fixed so quickly (or there is enough redundancy) that operations are barely affected. So, what’s the problem?
Most organizations fail to consider the costs associated with inventory which we file under the name “Inventory Carrying Costs.” These costs include storage, maintenance, manpower for counting and organizing, and more nebulous conditions like the weighted average cost of capital (including money tied up in inventory that could be lucrative in other parts of the business). These costs typically range between 20-30% of the inventory value on an annual basis. Does your organization have an “insurance mentality” when it comes to spare parts?
9- How to Determine Spare Parts
In the previous topic, we discussed that having one of everything “just in case” is not the way to stock a warehouse. So, what is? If we don't plan to simply fix things after they break, then we should probably fix things before they break (most of the time)! It all starts with identifying the appropriate maintenance strategy and developing maintenance tasks to prevent or mitigate the consequences of functional failure. For example:
- If the strategy is fixed time replacement, you should develop a repair bill of materials.
- If the strategy is condition directed, a planned corrective task with a repair bill of materials should also exist.
- If the strategy is no planned maintenance, there should be a planned corrective maintenance task with a repair bill of materials.
Notice this is the caveat I mentioned earlier; we do sometimes wait for stuff to break and then fix it, but it is only a strategy if we planned to do it!
These repair bills of materials establish the list of inventory parts, but not the order point or economic order quantity. To determine these, we use equations with inputs like lead time, part cost and frequency of use to mathematically determine when and how many parts to order. Are your stocking levels set correctly?