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Mean time between failures is a measure of an asset's reliability. It shows you how long, on average, an asset can run before you need to repair it. On a basic level, you can use MTBF as a maintenance metric to see how well your team maintains assets. 

But you can also use MTBF to improve your operations, including tightening inventory control and fine-tuning your preventive maintenance program. Let's start with some basic definitions. 

Definition of mean time between failures 

Mean time between failures is the amount of time, in hours, that an asset can run before it fails and the maintenance team needs to repair it. And the word "repair" is key here: you only calculate MTBF for assets you can fix. For things that you can only ever replace, for example light bulbs, you use a different maintenance metric. 

To get a better sense of what MTBF can tell us about our maintenance operations, it's worth untangling the relationships between MTBF, reliability, and availability. 

MTBF is closely connected to an asset's reliability and is a part of the reliability equation: Reliability = e(time/MTBF). That means the longer your MTBF, the higher your reliability. A longer MTBF also improves your availability. 

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But what's the difference between reliability and availability? Reliability is the chances an asset or piece of equipment can perform without failures. Availability is the chances that an asset or piece of equipment can perform at any given time. When you calculate availability, you need to take into account both its reliability and the amount of time the maintenance team tends to take to get it back up and running after a failure. 

Why MTBF is sometimes mean time between system aborts (MTBSA), mean time between critical failures (MTBCF), and mean time between unscheduled removal (MTBUR) 

Not all failures are equal, so MTBF has more than one variation, each with a different definition of failure. 

For example, there are complete failures, where the equipment is down and the maintenance team needs to repair it before it can go back online. Think of a car with a flat tire. There are also partial failures, where the equipment is still working but not at full capacity. Think of a car with a bad muffler; you can still drive it. A car with a broken radio antenna? That one component has failed completely, but the car runs fine, and you might not even notice the failure until you try to turn on the radio. 

How to calculate mean time between failures 

Start by tracking three numbers: the total number of hours the asset was in operation, the number of times it failed, and the amount of time it took to repair after each failure. 

Then take the total number of hours of operation (hours running minus the hours it was down and being repaired) and divide it by the total number of failures. 

Don't worry about downtime related to preventive maintenance work. In fact, don't include any time the asset was scheduled to be offline. For example, if your manufacturing plant is only running one shift, a press might only be scheduled to be online eight hours a day. Over the course of three days, that's 24 hours, not 72. 

Make sure you have a good definition of "online," too, because it's different for different assets. Take a hydraulic press, for example. Is it online the whole time it's simply on? Or, is it only really online when it's applying force? Another example is a forklift. Do you include all the time it's on or only the times it's carrying a load? 

There's no perfect answer, but you do need to make sure you're consistent. 

How to use the MTBF formula 

Let's look at a simple example. Say you have a motor that ran for 24 hours. During that time, it failed twice. Each time it took the team an hour to fix it. 

The motor ran for a total of 22 hours (24 hours minus the two hours it took for repairs). Twenty-two divided by two, the total number of failures, equals 11. 

Not a great asset. On average, it's going to fail every 11 hours. That's not good. Is it time to look for a new one? 

Why use MTBF software 

Have you already thrown out that press? That was maybe not the right decision. 

In some cases, a low MTBF means it's time to retire an asset and bring in its replacement. If an asset has been generally reliable but is now giving you more headaches, it might be close to the end of its useful life. But not always. 

A low MTBF can also come from operator error and poor maintenance practices. It's failing because of how the operators are running it and how the technicians are trying to keep it up and running. 

The first thing to check is that operators aren't abusing the equipment. If they are, you need to determine if it's from ignorance or indifference and then take the appropriate steps. If it's not operator error, you can start to look at your MTBF software for ways to streamline and strengthen workflows. 

With the right MTBF software, you can improve your maintenance programs, boosting your KPIs and maintenance metrics, including MTBF. 

Standardized work order processes and procedures 

With a team of techs, you can't always have the same person maintaining and repairing the same assets and equipment. It's a scheduling nightmare. So the question becomes: How can I make sure every tech does the work the right way, every time? 

With MTBF software, assigning work orders is much easier and much more consistent. Instead of trying to scribble notes onto an easy-to-lose slip of paper or cram everything into spreadsheet cells, modern work order management solutions make it easy to give techs everything they need to work fast and consistently, including: 

  • Comprehensive maintenance and repair histories 
  • Step-by-step instructions 
  • Customizable checklists 
  • Digital images, schematics, and O&M manuals 
  • Associated parts and materials 

Now, instead of techs relying on trial-and-error troubleshooting and plain guesswork, they have instant access to your current best practices. 

Balanced MRO inventory management and control 

That last item on the list, associated parts and materials, relates to another area where MTBF software helps. 

Once you know your MTBF for an asset, it's a lot easier to look at lead times and set par levels. Here's how that works: Once you know that one of your presses tends to break down once every three weeks, you can more easily plan which parts and materials you're going to need and when. If the press tends to need a new bolt, washer, or spacer, you can make sure you have them on hand by looking at your current levels and the lead times. Let's say you need one every three weeks, you have two on hand now, and the lead time is three weeks. As soon as you use one more, it's time to place an order. 

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With the right software solution, most of this process is automatic. Set your current and par levels in the software, and every time you close out an on-demand or preventive maintenance work order with that part or material, the software subtracts it from your current levels. When you hit the par, the software sends you a message letting you know it's time to reorder. It can even automate some of the purchasing steps. 

Perfectly timed preventive maintenance inspections and tasks 

And just like you can use MTBF to help you set up inventory control, you can also use it to schedule preventive maintenance inspections and tasks. If you know an asset, on average, fails after roughly 100 hours of operation, you can set PMs at every 85. That way, you're getting the most bang for your PM buck. 

MTBF tells you when the asset or equipment tends to fail, and a quick review of your data can tell you why it tends to fail. By looking at both, the why and the when, you can have the right set of inspections and tasks at the right time. 

Next steps 

If you're thinking of implementing a CMMS solution, or you already have one but it's failing to live up to what you were promised, now's the time to finally get the maintenance management system you deserve. By reaching out to providers and explaining your current challenges, you have the opportunity to learn more about the software and find out exactly which one is the right one for you .

Wayne Metro success story

About The Author

Jonathan Davis

Jonathan has been covering asset management, maintenance software, and SaaS solutions since joining Hippo CMMS. Prior to that, he wrote for textbooks and video games.
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