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Home » Articles » Page 247

Articles

Find all articles across all article series listed in reverse chronological order.

by Fred Schenkelberg 2 Comments

Can a Product Have Perfect Reliability?

Can a Product Have Perfect Reliability?

Perfect Reliability? The product lasts too long?

In the poem by Oliver Wendall Holmes, The One Hoss Shay, a deacon is confounded by the various parts of his carriage the fail.

And, he decides to do something about it.

But the Deacon swore (as Deacons do,
With an “I dew vum,” or an “I tell yeou,”)
He would build one shay to beat the taown
‘n’ the keounty ‘n’ all the kentry raoun’;
It should be so built that it couldn’ break daown:
“Fer,” said the Deacon, “t’s mighty plain
Thut the weakes’ place mus’ stan’ the strain;
‘n’ the way t’ fix it, uz I maintain, Is only jest T’ make that place uz strong uz the rest.”

Translating from old English, it basically means he wanted to craft a carriage using the best materials and techniques. Later, he built a very sound carriage where every part is just as strong as all the other parts. [Read more…]

Filed Under: Articles, Musings on Reliability and Maintenance Topics, on Product Reliability

by Fred Schenkelberg Leave a Comment

Retro Standard Deviation Calculation

Retro Standard Deviation Calculation

Edited by John Healy

You use your calculator or spreadsheet, or even a statistics software package to calculate standard deviation, which is an estimate of the population standard deviation. Yet, understanding how one could calculate standard deviation without such advanced tools may prove useful. The knowledge of basic sum of squares methods provides a foundation for ANOVA and DOE analysis techniques. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Probability concepts

by Fred Schenkelberg 1 Comment

Meditation and Design for Reliability

Meditation and Design for Reliability

Is it possible for an individual to ‘do’ DFR? Is design for reliability something, like a specific technique, that is DFR?

What is DFR and how would you recognize it if it was occurring? Like meditation, nearly anyone can strike a pose that appears similar to someone in deep meditation, yet can you tell by observation if they really are mediating? Probably not. The same is true for an organization or person that declares they are doing DFR. Maybe they are or maybe not.

[Read more…]

Filed Under: Articles, Musings on Reliability and Maintenance Topics, on Product Reliability

by Fred Schenkelberg 5 Comments

The Law of Large Numbers and the Gambler’s Fallacy

The Law of Large Numbers and the Gambler’s Fallacy

edited by John Healy

This theorem is a fundamental element of probability theory. The law is basically that if one conducts the same experiment a large number of times the average of the results should be close to the expected value. Furthermore, the more trails conducted the closer the resulting average will be to the expected value.

[Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Probability concepts

by Fred Schenkelberg 2 Comments

Is Environmental Testing Part of Product Reliability?

Is Environmental Testing Part of Product Reliability?

Environmental testing is the evaluation of a product or system in one or more stress conditions. Environmental as in that which surrounds and affects a product. Consider temperature. Is the product going to experience outdoor temperatures as found in Fargo, North Dakota or Belmopan, Belize?

The weather is one way to describe external stresses, yet it is so much more. Environmental testing may include fungus, insect, and animal exposure. The document MIL-STD-810G lists and describes testing methods for a wide range of environmental conditions. [Read more…]

Filed Under: Articles, Musings on Reliability and Maintenance Topics, on Product Reliability

by Fred Schenkelberg Leave a Comment

Reading a Standard Normal Table

Reading a Standard Normal Table

Editing and Contributions by John Healy

Before computers and statistical software, we relied on tables to determine values for common integration problems – the normal distribution in particular. There is no closed form solution for the integral of the normal distribution probability density function, it requires advanced numerical methods to estimate the area under the curve. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Statistics distributions and functions

by Fred Schenkelberg 5 Comments

When to Stop Testing

When to Stop Testing

Stop testing when the testing provides no value.

If no one is going to review the results or use the information to make a decision, those are good signs that the testing provides no value. Of course, this may be difficult to recognize.

Some time ago while working with a product development team, one of the tasks assigned was to create an ongoing reliability test plan. This was just prior to the final milestone before starting production. During development, we learned quite a bit about the product design, supply chain, and manufacturing process. Each of which included a few salient risks to reliable performance.

 

[Read more…]

Filed Under: Articles, Musings on Reliability and Maintenance Topics, on Product Reliability

by Fred Schenkelberg Leave a Comment

Central Limit Theorem

Central Limit Theorem

There are two basic ways to consider the central limit theorem. First consider a random variable, X, which has a mean, μ, and variance σ2. If we take a random sample from f(X) of size n and calculate the sample mean, X̄, then as n increases the distribution of the sample means, X̄’s approaches a normal distribution with mean, μ, and variance σ2/√n̄. The original data, X, may have any distribution and when n is suitably large the distribution of the averages will approach a normal distribution. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability

by Fred Schenkelberg 2 Comments

Reliability and Monte Carlo Determined Tolerances

Reliability and Monte Carlo Determined Tolerances

In the Monte Carlo method, one uses the idea that not all parts have the same dimensions, yet a normal distribution describing the variation of the part dimensions is not assumed.

Although the normal distribution does commonly apply, if the process includes sorting or regular adjustments or if the distribution is either clipped or skewed then the normal distribution may not be the best way to summarize the data.

As with any tolerance setting, getting it right is key for the proper functioning of a product. Monte Carlo method allows you to consider and use the appropriate models for the variations that will exist across your components. [Read more…]

Filed Under: Articles, Musings on Reliability and Maintenance Topics, on Product Reliability Tagged With: Tolerance analysis

by Fred Schenkelberg 5 Comments

The Normal Distribution

The Normal Distribution

A continuous distribution is useful in many statistical applications such as process capability, control charts, and confidence intervals about point estimates. On occasion time to failure, data may exhibit behavior that a normal distribution models well. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Statistics distributions and functions

by Fred Schenkelberg 2 Comments

Reliability and Root Sum Squared Tolerances

Reliability and Root Sum Squared Tolerances

The root sum squared (RSS) method is a statistical tolerance analysis method. In many cases, the actual individual part dimensions fall near the center of the tolerance range with very few parts with actual dimensions near the tolerance limits.

This, of course, assumes the part dimensions are tightly grouped and within the tolerance range.

Setting tolerances well, using the best available data about the part(s) variation, allows creating designs that function well given the expected part variation. This is better for reliable performance. Also, the same method can be applied when the loads and stresses are normally distributed.

Check that assumption with you data first, of course. [Read more…]

Filed Under: Articles, Musings on Reliability and Maintenance Topics, on Product Reliability Tagged With: Tolerance analysis

by Fred Schenkelberg 5 Comments

Upcoming live online CRE Prep course

Learn more and register for the CRE Prep course here. (registration is closed at this time)

Overview

  • This online class provides an overview of Body Of Knowledge with guided self-study by the student, review of previous CRE exam questions, hands on-line approach.
  • This virtual classroom allows participants to actively participate by drawing on the virtual whiteboard, asking questions on the mic or web chat, taking polls, sharing desktops, and webcam capability.  Imagine following a lecture, highlighting or circling your problem areas on the virtual whiteboard, or even presenting tough CRE questions to be answered.  You can even e-mail questions to be answered in the next class!
  • If you are unable to make the live class or want to re-view videos from past sessions, you may do so by clicking any course date in the Course Archive.
  • This course is designed to supplement the knowledge of the individual having met the requirements for certification and is not designed to teach the entire body of knowledge.

The course is 16 two hour sessions – a mix of lecture, discussion and Q&A that focused on what you need to know to pass the ASQ CRE exam. Sure, beyond the course you will need to study. This involved practicing sample exam questions, using your references and calculator, and practicing finding answers quickly.

We discuss ways to study along with any issues or ideas you have as you prepare for the exam.

*Note: These online classes will be Archived and you will be able to access them later even if you cannot make it to the Live class session!  This means you may watch them again in case you forget something, miss a class, or can’t make it one day.  Saturdays may be scheduled with the instructor for extra sessions or make-up classes if needed.  

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Looking forward to seeing you in the course. And, of course, at any time, feel free to ask questions here, or in the CRE Preparation LinkedIn group.

Learn more and register for the CRE Prep course here.

Filed Under: Articles, CRE Prep, CRE Preparation Notes

by Fred Schenkelberg 4 Comments

Lognormal Distribution

Lognormal Distribution

Similar to the Weibull distribution yet with slightly heavier tails. While not as easy to interpret if the data shows early life or wear out features, the lognormal distribution often fits time to repair data accurately.

Transform the data by taking the natural log of each data point. The resulting values tend to be normally distributed if the original data fits a lognormal distribution.

You can use base 10 or base 2 or any base and the results will still tend to be normally distributed. It is common to use natural log, ln(). [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Statistics distributions and functions

by Fred Schenkelberg Leave a Comment

Reliability and Worst Case Analysis

Reliability and Worst Case Analysis

Worst-case tolerance analysis is the starting point when creating a tolerance specification.

It is a conservative approach as it only considers the maximum or minimum values of part variation—whichever leads to the worst situation. Setting tolerances such that the system will function given the expected variation of manufactured components improves that ability of the system to perform reliably.

In the worst-case method, you simply add the dimensions using the extreme values for those dimensions. Thus, if a part is specified at 25 ± 0.1 mm, then use either 25.1 or 24.9 mm, whichever leads to the most unfavorable situation.

The actual range of variation should be the measured values from a stable process. It may be based on vendor claims for process variation, industry standards, or engineering judgment. [Read more…]

Filed Under: Articles, Musings on Reliability and Maintenance Topics, on Product Reliability Tagged With: Tolerance analysis

by Fred Schenkelberg Leave a Comment

The Poisson Distribution

The Poisson Distribution

A discreet distribution useful when counting events within a time period.

It models rates, such as the number of gophers in your garden, paint scratches on your car, or the number of shopping carts that arrive in the 5 minutes before you in line. Essentially it’s the count of something over a time period or defined area. [Read more…]

Filed Under: Articles, CRE Preparation Notes, Probability and Statistics for Reliability Tagged With: Statistics distributions and functions

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