Most plants around the world are filled with time based maintenance tasks. After all, it is the easiest way to do preventive maintenance. The problem is, these plants are not using time based maintenance effectively because they are using it for the wrong types of failure modes. As a result, they are wasting a lot of resources either over maintaining their equipment, or simply doing ineffective maintenance. That’s why in this article, I talk about what time based maintenance is, it’s relation to age-related failure modes, and a few situations in which it’s best to use time based maintenance.
Key points:
- Research shows that most failure modes are not age related. Depending on the industry and equipment, about 60% to 70% of your failure modes, maybe more, are not age-related.
- Time based maintenance aims to restore or replace a component regardless of condition. You restore or replace a component at a predefined interval, whether the component is in good condition or not.
- Time-based maintenance is only effective when you’re dealing with a failure mode that has a clear and well-defined wear out age.
- The consequences of failure and economic factors need to be considered when deciding on whether to choose time based maintenance or not.
Now, before we talk in detail about what time-based maintenance is, you first need to understand what age-related failure modes are and how they are best dealt with using time based maintenance.
What are age-related failure modes?
Age-related failure modes are failure modes that have an increased chance of failure as a function of time. So that could be early life failures, also known as infant mortality, or wear-out failures that occur later in life. As shown in the diagram below, early life failures have a failure pattern A and F, whereas wear-out failures would have a failure pattern like B or C.
As you can see, patterns A, B and F all have periods where the failure rate is significantly higher in the bulk of the time. And that is something we can potentially use to our favor when developing effective maintenance tasks. Pattern C is a wear-out pattern that shows failure rates steadily increasing over the life of the item. There’s no real break point after which the rate of failure significantly increases.
Although we have these different failure patterns, you must remember that most failure modes out there exhibit a constant probability of failure. Somewhere between 70% to 90% of our failures are not age-related. But you need to understand which failure modes are clearly age-related and which ones are not so that you can select effective maintenance tasks.
Early Life Failures (Infant Mortality)
Now let’s have a look at early life failures or infant mortality in a bit more detail. As we saw from the failure patterns, early life failures exhibit a decreasing failure rate until it stabilises. That means the chance of a failure is much higher just after start-up than after a sustained period of equipment being operational.
Early life failures are usually the result of latent defects from the manufacturing period, or the result of undetected damage that could have occurred during transport, installation, or commissioning. It is these defects or damage that then lead to early life failures shortly after startup.
Early life failures can also point to human error like incorrect design, incorrect selection, or maintenance induced errors. So, for example, if you find that some of your mechanical seals fail very early in life, that could be pointing to a design error where you have specified the wrong seal, or you have issues occurring during installation.
When it comes to managing infant mortality, that is best achieved through a rigorous design process, accompanied by robust quality control during fabrication, installation, and commissioning. Another option is soak testing where components are tested extensively before placed into operational service.
Wear out Failures
The opposite of infant mortality is wear-out failure, which occurs late in life. This pattern exhibits an increase in failure rate over time due to physical, chemical, or other type of degradation. Think of things like fatigue, corrosion, erosion, abrasion, loss of elasticity.
When it dealing with wear out, you first of all want to reduce the equipment’s exposure to the wear out mechanism if possible. And then you would typically look at component replacement or overhaul towards the end of the useful life. So we’re going to have a look at that in a minute, but first I want to just talk about one more thing. And that is, of course, that in the end everything wears out, and in the end everything fails. However, what we are discussing about here is whether the equipment exhibits a significant increase in failure rate during its useful life.
Time based maintenance and age-related failures
Now, let’s have a look at how we deal with these failure patterns from a maintenance perspective.
When you look at pattern B, you can clearly see that shortly after time, T, marked on the graph here, the likelihood of failure starts to increase significantly. So, if we know time T, then it is worth doing a time-based maintenance task. At time T, overhauling or replacing the items would be a very effective and efficient way to deal with this failure mode.
And when you look at pattern C, you see that there is no real rapid increase in likelihood of failure over time. Instead, it’s more of a steady increase. So, for this failure pattern, when would you do a time based maintenance task? It’s hard to say. That’s why you would typically not use time-based maintenance tasks if you were dealing with this pattern. Instead, condition-based maintenance is often a better option.
What is Time-based Maintenance?
Looking back at the definitions I introduced in the article “The 9 Types of Maintenance: How to Choose the Right Maintenance Strategy”, time based maintenance is one of the five types of preventive maintenance. It refers to replacing or renewing an item at a fixed time interval or usage, regardless of its condition.
As you have seen, Time-based maintenance is only effective when you’re dealing with a failure mode that has a clear and well-defined wear out age. If we know the time T, then we can conduct a time based maintenance task at that point.
Now the issue here is knowing when to make the intervention. Do it too early and you will not fully utilise the life of your equipment. You are in essence over maintaining and that could cost a lot of money. Do it too late and you’ve just incurred a functional failure. Now that could be okay, but the impact could also be very significant and it’s something you really want to avoid.
Unit of Measure
Now before we go any further, I want to point out that although we talk about time based maintenance, the unit of measure could indeed be like time, like days or weeks or months.
But it can also be usage related like kilometers or miles as it would be the case for your car, or the number of cycles that a machine goes through, or the number of starts and stops. Anything that can effectively describe the onset of wear and the increase in probability of failure.
Not all monthly tasks are time-based
Now don’t confuse a monthly inspection as a time-based maintenance task just because the task is done every month. Inspections are typically condition assessments where you look for signs of a deteriorating condition and tasks like these are really condition based maintenance.
Similarly, if you test a standby diesel generator every two weeks to make sure it’s starting, that is not time based maintenance, even though it’s done on a fixed time frequency. You’re testing your standby generator to see if it worked because you’re concerned about an engine failure mode. And therefore, this would be failure finding maintenance.
Benefits and Drawbacks of Time Based Maintenance
Now what are the benefits and drawbacks of using time based maintenance?
Benefits
Time based maintenance is relatively easy to establish as you simply conduct your task as a fixed interval, potentially as simple as every month or every three months. It doesn’t really get much easier than that. And because of that, and the fact that you conduct a task irrespective of condition—it becomes very easy to forecast your resources, your spare parts usage, and your budget. But most important, time-based maintenance can be highly effective and efficient use of your scarce resources when you’re using it for the right type of failure modes.
Drawbacks
One of the main drawbacks of time based maintenance is that it only applies to failure modes that are clearly age related. And research shows that most failure modes are not age related. Depending on the industry and equipment, about 60% to 70% of your failure modes, maybe more, are not age-related. Therefore, time based maintenance should never make up the majority of your preventive maintenance program.
Another key drawback with time based maintenance is that it can be quite easy to end up over maintaining your equipment. You see, predicting that time, T, where the likelihood of failure rapidly increases can be quite hard, and it’s not always the same either. So organisations tend to be conservative to avoid the risk of failure and therefore, they shorten that interval for the time based maintenance task. Before you know it, you’re doing that task every month instead of every quarter, which means you’re doing it twelve times a year instead of four times a year.
Now, let me give you some situations that you can think about whether you should use time based maintenance or not.
Examples
Lube Oil
Let’s consider lube oil. Do you think lube oil is best replaced say every two years or every 15,000 kilometers or every 4,000 hours? It is certainly a common practice and it can be an effective way of managing your lube oil.
But if you think about the potential failure modes that relate to lube oil like contamination or degradation of additives, those are typically not age-related failure modes.
Which is why many companies have adopted lube oil sampling and analysis, which in essence is condition based maintenance. You look at the condition of your lube oil monthly and you decide whether to make an intervention.
Now the beauty of this is that you can deal with random failure modes and you can typically run your lube oil for much longer than if you took a time based maintenance approach. And for big industrial equipment with very large volumes of lube oil, moving from time based maintenance to condition based maintenance through a sampling regime would save a lot of resources.
But if you have a fleet of relatively small machines with much smaller lube oil volumes, it probably does not make much economic sense to adopt a sampling regime. And instead, you would likely opt for a conservative change out frequency for the oil.
So to answer the initial question, do we manage lube oil with time based maintenance or not? The unfortunate answer is yes and no. Sometimes we do and sometimes we certainly should not.
Oil Filters
What about our lube oil filters or say, engine oil filters, or maybe even process filters? The same applies here, time based maintenance may or may not be the right choice. Let me explain a bit more.
Take an engine oil filter, that’s typically a cheap part that is very easily installed, so it makes sense to simply replace the filter irrespective of condition say every month. Now you could of course have a differential pressure measurement across the filter to assess its condition, but for many engine filters, it’s probably not worth the trouble.
But if you were to look at large process filters that are expensive and you potentially have dozens, maybe hundreds, of filter elements installed and it requires a lot of work to change, then you want to take a different approach. In those kinds of cases, you want to install a DP measurement across the filters, monitor the DP trend, and decide when the DP cross a certain threshold would trigger a condition task to go and replace those filters.
In that case, you want to move away from time based maintenance because you risk over maintaining and you would incur a lot of extra costs that is probably not worth it.
But once again, it depends on whether you’re dealing with small, easy-to-change, low-cost filters or complex, expensive, a lot of effort to replace, big process filters. So once again, the answer is that it depends.
Conclusion
So, in summary, when should you use time-based maintenance?
The obvious and dominant reason to use time based maintenance is when you have a failure mode that has a clear and well-defined wear out age. A threshold that you can express in some kind of usage or time measure after which the likelihood of failure significantly increases.
Now if the wear out age is unpredictable, the cost of maintenance is very high, or the consequence of failure is very high, then you want to avoid using time based maintenance because you will either waste a lot of money or you will risk experiencing significant failures.
In these circumstances you’re most likely better off opting for condition based maintenance tasks. That said, if the wear out age is unpredictable, but the cost of maintenance is low when a consequence of failure is low too, well then you may decide to opt for time based maintenance and accept that you are either over maintaining slightly or will sometimes experience failure.
