Given MTBF? Now What?
Let’s say you join a project as a reliability professional (or an engineer or manager of any type) and you discover that the team as a reliability goal stated as 5,000 hours MTBF.
What are you going to do with that information?
Talking about Reliability
How do you talk about reliability?
“The language we use matters.” Wayne Nelson
When we talk about our products or equipment, we may refer to the expected durability of the system.
- How long it will work before failure?
- How long before we have to make repairs?
- Will it work when we need it to work?
Our customers and investors also want to know how long will it last.
Expecting MTBF
What to Expect from MTBF
What do we really want?
When using the term, MTBF, many believe they are talking about the reliability of a device or system. A high MTBF numbers means it is a reliable item.
What we really want is the device to work over some duration without failure (or with few failures). It should perform a function as expected in the desired environment.
Questions to ask a vendor
How do we get reliability information from vendors?
One of the major risks for product reliability is bad components. Either not suited for the system or adverse variation over time from the supplier.
Reading data sheets and hoping for the best is often all we have time to accomplish. For high risk (new technology, new supplier, new… something) many organizations will examine the reliability claims to some extent. Some may even conduct reliability testing in-house to validate vendor claims.
When a customer wants reliability
As Reliable as the Sun and the Moon
What do customers want when they say they want reliability?
Maybe it is that the product will work when they need it to work.
Like your car starts in the dark parking garage after a long snowy day so you can head for home. The goal is getting home.
Provide value when talking about reliability
Value in being clear about reliability
One of my regular questions with clients and students is, “How do you talk about reliability, which metrics, measures, or statements do you regularly use?”
Some have learned to avoid mentioning MTBF around me. Which is fine, and if they are really using MTBF to discuss reliability then they probably know my position.
A no MTBF standard?
Is there a No MTBF standard?
Sure, don’t use MTBF.
You can use that as a standard or directive or policy or however you want.
The IEC set of durability standards have been slowing shedding reference and use of MTBF. That is a good thing and I thank them for their leadership.
Solving Type III problems
Solving Type III problems
There are occasions when we perfectly solve the wrong problem. This is a Type III error.
Following the statistics idea of Type I and Type II errors, when a sample provides information incorrectly about a population, Type III is the error of asking the wrong question to start.
Solving the wrong problem, even perfectly, is still an error.
So, how do you know you’re in a Type III situation?
MTBF and Vacation
Does MTBF make any sense?
My wife and I are just wrapping up a two-week trip to New Zealand. As I write this I’m overlooking the lake near Queenstown.
There are parasails floating down from the hill, mountain and road bikers on the trails and roads, trampers, hang-gliders, jet boating, sailing, and bungy jumping – and dozens of other activities occurring.
6 ways to overcome MTBF stubbornness
6 ways to overcome MTBF stubbornness
Just before making an 1 hour presentation at a reliability engineering conference George asked me how to teach others what MTBF really means.
Not having given this much thought before, I asked for more details.
George works as the sole reliability engineer for a small company making specialized networking monitoring equipment.
How do you talk about reliability?
How do you talk about reliability?
How do your customers? And, your suppliers?
If they are all using MTBF, there is a pretty good chance one or more parties involved have some misunderstanding of MTBF. It’s common for some to think it is a failure free period. Or they don’t count early life or wear-out type failures when doing the calculation.
All models are wrong
but, some are useful
As said by George E. P. Box. He was talking about statistical modeling and the basic idea behind actually doing the modeling.
We want to make decisions.
MTBF and MTTF Definition(s)
Recently Glenn S. asked if I had a reference for clear definitions of MTBF and MTTF. After a bit of a search I sent him a definition or two, meanwhile he gathered a few more.
They are all basically the same, with some slight differences. What is interesting to me is the amount of variability in the interpretation and understanding.
Here’s the list Glenn collected:
Understanding the Limitations of MTBF
A Brief Look at Understanding the Limitations of MTBF
Mean-Time-Between-Failure (MTBF) as defined by the MIL-STD-721C Definition of Terms for Reliability and Maintainability (12 June 1981) is
[a] basic measure of reliability for repairable items: The mean number of life units during which all parts of >the item perform within their specified limits, during a particular measurement interval under stated >conditions.
MTBF is widely used to describe the reliability of a component or system. It is also often misunderstood and used incorrectly. In some sense, the very name “mean time between failures” contributes to this misunderstanding. The objective of this paper is to explore the misunderstood nature of MTBF and its impact on decision making and program costs.
Calculating Capacitor Reliability
Objective
Based on vendor data and engineering estimates, calculate capacitor reliability for use in system modeling.
Components
For a large capacitor in the system determine the expected life distribution for use in the system reliability block diagram model. The capacitors is:
DC Buss capacitor, mfg p/n E50.R16-155N10; CAP FLM 1450uF 700VDC 10%0.65Rs 40nH 116Dx165H -25+85C ROHS
From the datasheet
statistical failure rate 50 FIT*
reference service life 100000 h
at θhotspot ≤ 70 °C
*FIT is a function of temperature and voltage
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