Ep 3: What is Preventive Maintenance and how are task intervals assigned?April 13, 2022 April 19, 2022 / Published By
In this episode, we talk about what preventative maintenance is and what two criteria determine if a preventive maintenance task should be assigned. The biggest pitfall that organizations often fall into when defining intervals for preventative maintenance tasks is revealed.
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Hi, everyone. Welcome to The Heart of Reliability, where we break Reliability down to the basics because after all, anything that’s not built on a strong foundation will eventually crumble
I’m Nancy Regan. And welcome to episode three of The Heart of Reliability. Today, we are going to talk about Preventive Maintenance. Specifically, we’re going to talk about:
– What Preventive Maintenance is
– What two things determine whether or not you assign a Preventive Maintenance task
– One of the biggest pitfalls that organizations often fall into when defining intervals for Preventive Maintenance tasks.
Okay, let’s get down to it. What is Preventive Maintenance? Well, technically Preventive Maintenance is a Scheduled Overhaul or another term for that is a Scheduled Restoration or a Scheduled Replacement task.
Now that term Preventive Maintenance within our industry is used a lot to define Proactive Maintenance in general. But technically speaking, Preventive Maintenance only encompasses Scheduled Restorations and Scheduled Replacements. So that’s the topic of this episode today.
Let’s talk about the two things that dictate whether or not you do a Preventive Maintenance task.
1. You have to identify if doing the Preventive Maintenance task is technically the right thing to do.
2. Is it worth it?
Let’s explore that first topic. Is it the right thing to do? I episodes one and two, I talked about my 2014 Subaru Forester, and I’d like to go back there to illustrate this point. Let’s start by identifying and talking about a failure pattern. Now, if you’re listening to this podcast and not watching it, what I’m doing is I’m drawing a failure pattern.
Imagine we have an X and a Y axis where the X axis is age, and that age can be calculated in any number of units. That could be calendar time. It could be operating hours, miles, or cycles. It doesn’t matter what it is. That’s the X axis.
Now imagine that the Y axis is the Conditional Probability of Failure. So the whole point about Preventive Maintenance is that we take action before failure occurs. But we do it on a specified interval. Now, anytime we do maintenance we are managing a particular Failure Mode. So, in this example using my 2014 Subaru Forester, let’s say that my Failure Mode is engine oil deteriorates due to normal use.
If I have my oil replaced today, I know that after a certain period of time, additives are going to be depleted and properties are going to break down. And if I continue to drive, the oil is not going to be able to lubricate my internal engine components properly. And over time, that’s going to do damage to my engine and that’s not going to be good.
So, if we if we look at this failure pattern, and if we start right at the point, the first point on the Y axis, we can see that there is a constant probability of failure. In other words, there’s a straight line, right? So, if you’re just listening, there’s a straight line, which means that my oil is good to go until I get to this interval for me, and for my 2014 Subaru Forester, this interval is 7,500 miles. So, I know that if I drive to 7,500 miles, if I keep driving after that, the probability that might oil is going to fail, assuming it makes it to 7,500 miles starts to drastically increase. And that’s where if you’re listening on this curve, imagine we see the line steadily going up, up, up, up, up. This is what governs whether or not you do a Preventive Maintenance task.
So this interval from the point where I put new oil into my engine, until it gets to 7,500 miles, this is called the useful life. My oil has a useful life. Now I’m not talking about an average life. I’m talking about the useful life. There’s a big difference between the two. We’ll get there, but let’s stick with this right now.
So, one of the technical criteria for assigning a Preventive Maintenance task is does it have a useful life? In other words, is there an identifiable wear out age?
Well, in my 2014 Subaru engine oil, there is. I know it’s 7,500 miles. So, if I go beyond this point, I know that my oil is going to break down. So, I take action. And what that action is, is that at 7,500 miles, I change the oil. And that brings me back to the beginning of my failure curve, where my oil is good to go.
Now that brings us to one of the biggest pitfalls when it comes to assigning maintenance tasks, any maintenance tasks, but today we’re talking about Preventive Maintenance tasks, and that is these four letters, M T B F or in other words, mean time between failure. MTBF by definition is an average. And an average, by definition, means that some failures will occur before that average life and some failures will occur after that average life. That’s the definition of average.
So to assign maintenance intervals based on an MTBF or a meantime between failure or an average makes absolutely no technical sense at all. So, when you are assigning Preventive Maintenance tasks, whether it is a Scheduled Replacement or a Scheduled Overhaul, it is absolutely vital that you identify what that useful life is.
So, the thing about Preventive Maintenance tasks – about Scheduled Restorations and Scheduled Replacement tasks is that they are done regardless of the items condition at the time. When I bring my Forester into Subaru every 7,500 miles, they don’t do oil analysis on it. They, they change the oil; they don’t check it first.
And that’s the nature of Preventive Maintenance tasks. It’s one of the most simple kinds of maintenance tasks to understand the theory behind it and to actually assign it. So, there it is, you do it regardless of the condition at the time.
Now, as responsible custodians, if we’re going to assign a Preventive Maintenance task, the other thing we have to consider is, is it worth doing? Because just because something has a useful life, it doesn’t necessarily mean that we should do the maintenance task. We have to figure out if it’s worth doing or not. If something does have a useful life, and it has safety consequences or environmental consequences, that’s a no-brainer. We don’t need any kind of an analysis to decide if it’s worth doing or not.
We absolutely need to do it if doing it at that interval does reduce the risk of failure to an acceptable level to the organization. But what if it just has economic consequences? Let’s go back to the example of my engine oil. If I bring my car in to have the oil changed every 7,500 miles, that means I’m bringing it in, say roughly twice a year.
And let’s say it costs me $50 to have the oil changed. Over a period of one year, it’s going to cost me about a hundred dollars. And over a period of 10 years, it’s going to cost me a thousand dollars to have the oil changed. Is it worth it?
Let’s say I never changed the oil. Maybe I just top it up every so often? Well, over time, we know what’s going to happen. That’s going to cause internal damage on my engine components. They are going to wear abnormally, and eventually it’s going to cause significant damage to my engine.
When that happens, it’s definitely going to happen before that 10 year period. What’s going to happen is I’m probably going to have to have some serious repairs on the engine or, worst case, have the engine replaced. Now, how much is that going to cost? That’s going to cost thousands, right? $5,000 +, not to mention the inconvenience of now not having my car.
I’m going to have to pay for a rental car and it’s going to be a hassle to have it done, right? So it’s going to cost me way more. Let’s say it’s going to cost me at least $5,000. Plus, it’s going to be a big headache. So, is it worth doing a thousand dollars over 10 years or $5,000 plus repairs plus a big pain in the butt?
In this scenario, it is obviously worth doing when it comes to Preventive Maintenance. Preventive Maintenance does apply to simple items – items that come into direct contact with a product like for example, engine oil and filters and structure. So that’s when we can apply this philosophy of a useful life and doing a maintenance task regardless of the condition at the time.
So, there you have it. We have established what it means that it is technically the right thing to do a Scheduled Replacement task and if it’s worth doing. And we’ve talked about one of the biggest pitfalls when it comes to Preventive Maintenance, and that is meantime between failure (MTBF) can be used to determine if it’s worth doing a maintenance task or not, but it has absolutely nothing to do with how often a Preventive Maintenance task is done.
What dictates how often we do a Preventive Maintenance task is the useful life. How long is something going to last until the conditional probability of failure starts to drastically increase? See, that’s the difference between probability of failure and conditional probability of failure. If we look at our curve again, what we say is when it comes to my engine oil, on the condition that it reaches 7,500 miles, isthat the probability of failure starts to drastically increase. See that’s the conditional probability of failure. It’s on the condition that it reaches a certain point.
Now you may have some Preventive Maintenance tasks in your maintenance plan, right? Maybe you change oil filters on a scheduled basis. That doesn’t mean that you can’t do Condition-Based Maintenance on them. And we’re going to talk about Condition-Based Maintenance in episode four of The Heart of Reliability, which is the very next episode. So, if you loved this one, stay tuned for the next one.
But when it comes to Preventive Maintenance, Scheduled Replacements and Scheduled Restoration tasks, what matters, what dictates the interval is the useful life. In other words, how long will it last until we know failure is going to occur.
So, there you have it. That is Preventive Maintenance in a nutshell. Those are the basics of Preventive Maintenance. And that’s what we’re all about here at The Heart of Reliability is getting down to the basics because when we understand the basics, when it comes to anything as human beings, when we have a firm foundation, then we’re able to build on that knowledge.
Thank you so much for joining me for this episode of The Heart of Reliability. I look forward to welcoming you back to the next one. Episode four is going to be all about Condition-Based Maintenance.
I’m Nancy Regan. Thank you for watching!
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