The Evil Ways of MTBF with Fred Schenkelberg

It’s my pleasure to welcome Fred Schenkelberg back to the podcast. He is a reliability engineer and management consultant.

What is MTBF?

It’s short for meantime between failures. Fundamentally it’s just an average of a failure rate over some duration inverted.

Why would people want to calculate what that average time between failure is or where did this come about?

It came from this early need to do calculations and estimates that were otherwise very tedious to do using the tools of the day. Over the years MTBF and MTTF is propagated because it’s dirt simple to do.

What do you think about using MTBF to monitor a complex systems reliability, and using that to pro proactively look at reviewing the maintenance program to it?

If you’ve got something that’s got early life failures, don’t replace it on schedule. If you got something that’s going to wear out, replace it before that risk of its failure gets too high. If you have something that has a relatively flat or a constant failure rate, you just wait till it fails and you replace it.

So, we end up with this MTBF for 10 years. Does that mean we’re going to go in and change all these bearings at year 10 or year nine and a half? What about the ones that we’re not installing correctly?

Stop using MTBF. It is just not useful. MTTF only fails once we have complex systems repair. There are so many other tools out there. If you have a system that has a spade of early failures, once you fix that and the system starts running well, the MTBF will get lower. If you’re counting on MTBF and you’re getting what appears to be worse, although you’re making progress, that’s frustrating to start with. And so, you will stop fixing early failures because the more of those you have, the lower your slope and the further out your end.

What is the goal of measuring MTBF?

It’s not the tool for everything. Do a histogram. Get your data and make a histogram which is much more informative than doing the average. My first step is just plot. Create a plot. Histogram is a great place to start. Learn a little bit more, go to the next page on the stats book that you put down years ago.  We’ve got great tools out there. We got great software packages and there’s enough people out there who have got webinars and podcasts and articles on these topics, you can learn how to do this.

Any other partying on MTBF?

Don’t use it. It’s useless. if you don’t believe me, go to no mtbf.com and there’s all kinds of fun stuff there to support this. It is not a good metric.

Anything you got coming up in terms of webinars?

Chris Jackson and I are putting him out about one every two weeks roughly. Chris will be talking about capability analysis, last month he talked about SPC and kind of the fundamentals of how and why that works. We search 30 to 40 different sites that do reliability related webinars, and we send out an email. If you can get on that list and you get one email, you get the summary of all the events that are going on in 30 different organizations.

Eruditio Links:

• Eruditio
• HP Reliability
• James Kovacevic’s LinkedIn
• Reliability Report

Fred Schenkelberg Links:

• Fred Schenkelberg LinkedIn
• Reliability.FM
• NoMTBF.com
• Use the Maintenance Data eBook
• Reliability Engineering in a Maintenance Environment

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