Your Reliability Engineering Professional Development Site
A listing in reverse chronological order of articles by:
“Aircraft LRUs test NFF (No-Failure-Found) approximately 50% of the time” {Anderson] Wabash Magnetics claimed returned crankshaft position sensors had 89-90% NTF (No-Trouble-Found), Uniphase had 20%, Apple computer had 50% [George].
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As the quality manger for a tier two automotive supplier, I recently had the opportunity to hire a quality supervisor following the retirement of a long-time member of our team. Our company’s human resource manager and I worked together through the entire selection process. Given the status of our region’s economy and the recent closure of several large factories, I wasn’t surprised when our mailbox started filling up with resumes in response to ads on the popular Internet job sites. The typical respondent was a mid-career professional with over 15 years of experience in manufacturing that had either been recently laid-off due or who wanted to move up in their career.
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How is failure testing done on the Space Station? Could FTA (Fault Tree Analysis) be used in reverse to detect multiple failures given symptoms? That’s what NASA was programming in the 1990s. I proposed that the ratios P[part failure]/(part test time) be used to optimally sequence tests. Those ratios work if there are multiple failures, as long as failure rates are constant and failure times are statistically independent.
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Abraham Lincoln taught the value of adequate preparation when he said, “Give me six hours to chop down a tree and I will spend the first four sharpening the axe.” By training, quality professionals are often focused on verifying the correctness of a product. A traditional inspector at the Lincoln Timber Company might have dutifully marked in her audit log the date and time, the type and size of tree, followed by the comment, “Cut down.”
But Honest Abe would have advised her to take a closer look at the tools and process used to complete the job.
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Shotgun repair is trying to fix a system problem by replacing parts until the problem goes away. It is often done without regard to parts’ age-specific reliability information. Should you test before replacement? Which test(s) should you do? In which order? How long? Which part should you replace next if the test gave no indication of what’s wrong? What if test indication is imperfect or the fault is intermittent? What if there are more than one part failure?
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“A genuine leader is not a searcher for consensus but a molder of consensus.”
Martin Luther King
One of the challenges for any team leader is bringing the team together and agreeing on decisions and actions going forward.
Consensus building is the best practice for all of the FMEA team decisions. This means the FMEA team takes the time to understand all sides of an issue and finds a solution or determines a course of action that is supported by all team members. Facilitating is a consensual activity.
With all the buzz these days about design thinking, some of you may be wondering what it’s all about. How does it relate to design? And what can non-designers gain from it?
Design thinking is far more than simply designing products and services—it’s an approach to problem-solving that can be applied to an incredibly wide range of applications.
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Which of these six failure rate functions do your products and their service parts have? You don’t know? You don’t have field reliability lifetime data by product name or part serial number? That’s OK. Lifetime data are not required to estimate and classify failure-rate functions, including attrition and retirement. GAAP requires statistically sufficient field reliability data to classify failure rate functions for RCM.
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COVID-19 Case Fatality Rate (CFR) is easy to estimate: CFR=deaths/cases. Regression forecasts of COVID-19 cases and deaths are easy but complicated by variants and nonlinearity. Epidemiologists use SIR models (Susceptible, Infectious, and “Removed”) to estimate Ro. These are baseball statistics. Reliability people need age-specific reliability and failure-rate function estimates, by failure mode, to diagnose problems, recommend spares, plan maintenance, do risk analyses, etc. Markov models use actuarial transition rates.
[Read more…]“Once you make a decision, the universe conspires to make it happen.” – Ralph Waldo Emerson
There are times when an FMEA facilitator has difficulty arriving at consensus with the FMEA team. This sometimes happens when there are two or more competing ideas or solutions and members of the team feel strongly about their personal idea or solution. One tool that can be used to solve this problem is called Pugh Analysis, a type of decision matrix.
Age-specific reliability of a standby system depends on components’ failure rates. Reliability computation is interesting when part failure rates depend on age, which is what motivates having a standby system. A Markov chain, approximates the age-specific reliability and availability, which are complicated to compute exactly, unless you assume constant failure rates. Why not use age-specific (actuarial) rates? They are Markov chain transition rates.
[Read more…]Mediocrity can talk, but it is for genius to observe. – Benjamin Disraeli
The Inside FMEA series has completed the primary facilitation skills. The next few articles will cover special facilitation topics.
This article talks about the pros and cons of using a “scribe” to help with facilitation. [Read more…]
Suppose installed base or cohorts in successive periods have different reliabilities due to nonstationarity? What does that do to forecasts, estimates, reliability predictions, diagnostics, spares stock levels, maintenance plans, etc.? Assuming stationarity is equivalent to assuming all installed base, cohorts, or ships have the same reliability functions. At what cost? Assuming a constant failure rate is equivalent to assuming everything has exponentially distributed time to failure or constant failure rate. At what cost?
In the 1960s, my ex-wife’s father set safety stock levels and order quantities for Pep Boys. He used part sales rates and the Wilson square-root formula to set order quantities.
Why not use the ages of the cars into which those parts go, to forecast part sales and recommend stock levels? Imagine you had vehicle counts (year, make, model, and engine) in the neighborhoods of parts stores, catalogs of which parts and how many go into which cars, and store sales by part number.
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The definition of the term “reliability” begins by specifying that reliability is a probability. Therefore, the concept of a probability, while sometimes intimidating to reliability practitioners, is fundamentally important.
Probability can be defined as the extent to which an event is likely to occur. Just as an average is a measure of central tendency, probability is a measure of uncertainty in a particular event or outcome. That’s all … Nothing intimidating here.
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