3 MTBF Stories
Everyone loves a great story. Storytelling has been a long tradition to pass along knowledge and wisdom.
There are good stories, tales of inspiration. There are sad stories, tales of caution.
There are fables, ghost stores, legends, epic poems, and more. When considering the reliability performance of your product or equipment, you probably have a few stories that you can tell. “That time … “
Simple join colleagues for lunch and ask about the ‘major disasters’ of the past. The stories help us to remember and hopefully avoid repeating mistakes.
Here are three stories with MTBF as a central figure. It is a site and blog that does take about MTBF, so it fits. To start, let me introduce you to Martin, a new reliability engineering reporting to his first day of work at a bicycle design and manufacturing company. Two sad stories and a good one. enjoy.
“We Only Use MTBF” – or the head in the sand story
As a former manufacturing engineer and with significant design for reliability accomplishments, Martin joined the ABC Bike Co as their reliability engineer. Martin knows about many of the tools within reliability engineering such as FMEA, HALT, ALT, modeling, failure analysis, etc.
Martin asked the director of engineering, Jas, for access to the production line’s reliability data. Jas says, “Sure, we have a lot of reliability data. We track every incident of downtime across the various product lines. Here are MTBF values by machine or station. Here are MTBF values per shift. It’s all here in the MTBF spreadsheet. Enjoy.”
With a little study and investigation, Martin learns that the data collection is simply a count of the downtime incidents by piece of equipment and by shift. The shift data is for the 8 hours of the shift across all the production lines and equipment. The equipment specific MTBF values does not indicate over what duration the data corresponds.
Back to Jes, Martin asks, “Why is the reliability data only MTBF summaries?”
Jes responds, “It’s what we use. We only use MTBF, it is for repairable systems. We don’t use MTTF as that is strictly for MTTF. Why, what else do we need to know?
At this point, Martin launches into a quick description of time to failure data, how failure rates change over time, and how MTBF is not helpful. He realizes the data collection practices will need improvement, plus some education.
“Customers Ask for MTBF” – or we don’t understand our product’s reliability story
Martin knows the bicycle lines enjoy a poor reliability reputation in the market. He heads over to see the director of engineering, Kyle. Kyle quickly explains the current design practice that focuses on creating durable, rugged bicycles, yet suspects the manufacturing and supply chain cost-cutting erode the ability to keep warranty expenses low.
This all seems good and the discussion turns to design for reliability. Martin asks, “What is the reliability goal for product xyz?”
Kyle says, “It is a difficult goal of 50,000 hours MTBF, a slight tick up in reliability from the previous model.”
Martin then asks, “Ok, over what duration? And, how many should survive that duration?”
Kyle looks a little confused as he responds, “50k hours, that is how long the bike should work without failure for a customer. Yet we seem to get plenty of failures prior to 5 years.”
“Why do you set the goal using MTBF?” (Martin)
“Because that is what our distributor ask for and expect from us. And, it’s the most common reliability metric right? Besides looking at our field data we are just about meeting the 50k goal now.”
Martin then scratches his head and wonders how much time they have left to discuss reliability goals, MTBF, and how successful they have been achieving their stated goal. Seems another round of education is in order.
Martin saved exploring product reliability testing practices for another day.
“We Don’t Use MTBF” – my favorite story
Let’s fast forward about 2 years. Martin is still working at ABC Bike Co and enjoying going to work each day. He’s accomplished quite a bit in the two years. He recently received a bonus for his contributions which cut warranty expenses to half what they were before he started working there.
His recognition by management for his achievements if great, yet the general greeting he receives walking through halls is wonderful praise, “We don’t use MTBF here.”
Martin changed the way the organization talks about reliability. It avoids confusion and misunderstanding. Using reliability, R(t), allows them to get the data that provides a clear understanding of changing failure rates over time. The entire organization is making better decisions and not using MTBF at all. The end.
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