Your Reliability Engineering Professional Development Site
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.
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.
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.
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 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.
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Cheers,
Fred
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.
So this corrosion engineers walks into NoMTBF and send me a message.
Hi, I am corrosion engineer. May be you know for risk assessment of heat ex-changer tube bundle in API-581 , mean time to failure (MTTF) term is defined and used for risk assessment.
Would you please give me more information about MTTF and what history data required to calculate MTTF?
Thank u so much
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:
This is a work in progress and – needs work.
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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.
Based on vendor data and engineering estimates, calculate capacitor reliability for use in system modeling.
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
Guest Post by Daniel Conrad, Director Design Quality, Reliability and Testing, Hussmann Corporation
Quality has always been important to the consumer, but with the current economic conditions, it is perhaps more important than ever. “Value” is today’s consumer buzzword, and that means designing products that not only meet a certain price point, but offer what consumers perceive as quality. Put simply: They want to invest in a reliable product.
But what exactly does that mean? Reliability is a concept, an expectation, a philosophy, and a metric. It means many things to many people. This is our challenge. Webster defines reliability as
the quality or state of being reliable; the extent to which an experiment, test, or measuring procedure yields the same results on successive trials.
I view reliability as engineering metric that defines the probability of a function’s success at a specific time under defined conditions. This is needed by engineers to ensure consistent measurements. I view “Design for Reliability” as the process to ensure the design is meeting the customer’s expectation or consumer-based reliability metrics. As quality professionals, I feel we need to help our organizations harness the power of these reliability concepts for the betterment of our customers, our organization, our shareholders, and other stakeholders.
Why do we care about reliability? The short answer for the producer and the consumer is money. For producers, statistics indicate that companies can spend up to 25% of their development budget solving reliability issues, according to www.conekt.net. Thus, a significant resource potential is available if we do it right the first time. Wouldn’t we rather put that money toward innovation or other business needs? For consumers, it is a matter of value. Do we get the benefits for which we paid? This is the heart of consumer-based reliability.
Consumer-based reliability is simply the delivery of the reliability expectation of the customer as part of the value equation. For consumer products, this can be a challenge for engineers to quantify due to the language. The customer’s understanding of durability, reliability, and quality make these terms nearly interchangeable.
From consumer surveys, the definition of durability was “reliable for a long time,” while the definition of reliability was “works for a long time.” However, poor reliability or durability has a direct relationship to a poor perception of brand quality. Additionally , there is a perception that new products are less reliable and less durable than previous generations. This is an artifact of consumer marketing and improved design methods. Clearly, part of the perceived decrease in life expectancy is driven by the harmonization of the appliance and electronics industries. The typical design life of a consumer appliance is 10 years today, but the life expectancy is dependent upon consumer use and care. This is why you hear the stories about an appliance working after 40 plus years.
Today’s appliances have been designed to eliminate the failure modes and mechanisms of past designs. For example, porcelain tubs that used to have pinhole leaks due to rusting are now plastic and no longer leak. As a result, this updated design has different failure modes and less heft. However, heft is what the consumer translates into durability and reliability. This shift in technology becomes a challenge and an opportunity for the designer. It allows the designer to shift the story to the relevant customer issues and how they are delivering consumer-based reliability to meet the customer’s needs.
In consumer durables, customer reliability expectations are a top consideration in a product’s perceived value, but customers do not expect perfection. They will tolerate a failure, but how that failure is handled matters: When the failure occurs, how much did it cost the consumer, and did the producer stand behind the product restoration? In addition, most consumers replace their appliances simply because the old one has died. It is important that this happens in a timeframe the consumer expects and accepts.
This brings us to the ultimate goal — customer satisfaction, which hopefully breeds brand loyalty . And in today’s market, everyone wants loyal customers. Today’s consumers are looking for branded products that will deliver value over a period of time — the true basis of reliability. And they demand it because they get it. The reliability of many consumer products has increased over the last decade — cars now routinely go 10 years and 100K miles without any major repairs and electronics like cell phones, TVs, and computers perform quite dependably and are usually replaced before their end of life. These every day experiences allow the consumer to demand the same dependability in all products.
Every satisfied consumer counts — as does every unsatisfied consumer. A satisfied consumer tells about four other people about his or her experience, while an unsatisfied consumer tells about 12 other people. This means the negative news “travels” three times faster than positive news. And thanks to the proliferation of web sites like You Tube, Garden Web, and Consumer Reports — not to mention product reviews from online retailers like Amazon — customers can quickly obtain reliability information and customer opinions on a product or service. A single consumer with a perceived reliability issue can communicate to millions of our potential customers with or without cause at very l ittle cost today. Blogs, consumer web sites, Twitter, and Facebook are global communication portals; therefore, “good enough” is not good enough anymore. We have to strive to meet the expectations of the consumer in a global interconnect “market space,” and consumer-based reliability is one aspect of meeting these expectations.
To do this the designer must become a customer advocate and translate the wants and needs of the customer into relevant reliability and durability specifications so that engineering can build the designs to meet these needs. Traditional reliability aims to be better than your competitors. Consumer-based reliability aims to meeting the customer expectations, which, if done right, will automatically beat out the competition.
About the Author
Daniel C. Conrad is the Global Engineering Director Design Quality, Reliability and Testing at Hussmann Corporation in St. Louis, MO, US. Prior to this he was the Global Design for Reliability Leader at Whirlpool Corporation in Evansville, IN, U.S.
He has a B.S. from the University of Kentucky in Mechanical Engineering and a MS and Ph.D. from Purdue University in Engineering Mechanics. He is a certified reliability engineer, is a Six Sigma black belt, and holds several U.S. patents.
As annual milestones occur, like the approach of the new year or the end of a project, we often take time to reflect. Did we accomplish what we had set out to accomplish? Are we making progress? Are we making a difference?
Part of this process is having a goal to start.
All models are wrong, some are useful. ~ George E. P. Box
If you know me, you know I do not like MTBF. Trying to predict MTBF, which I consider a worthless metric, is folly.
So, why the article on predicting MTBF?
Predicting MTBF or creating an estimate is often requested by your customer or organization. You are being specifically asked for MTBF for a new product.
You have to come up with something.