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Fred Schenkelberg — Thought Leader

Author of CRE Preparation Notes, Musings", NoMTBF, multiple books & ebooks>, co-host on Speaking of Reliability>/a>, and speaker in the Accendo Reliability Webinar Series.


This author's archive lists contributions of articles and episodes.

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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…]

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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…]

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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…]

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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.  

Mondays, July 13 – August 17, 2015 (3:00PM-5:00PM PDT)

Last Session for questions, September 21 and 28, 2015 (3:00PM-5:00PM PDT) 

This class ends prior to the semi-annual ASQ Section testing, where everyone may sign-up through their local Section to take their test.  Be sure to check out the ASQ website to get the early-bird rate before the end of this course!

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.

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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…]

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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…]

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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…]

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The Exponential Distribution

The Exponential Distribution

The exponential distribution is a model for items with a constant failure rate (which very rarely occurs).

If the chance of failure is the same each hour (or cycle, etc.), including the first hour, 100th hour, and 1 millionth hour or use, then the exponential distribution is suitable.

Many practitioners assume the failure rate either is constant or changes so little as to be essential constant in order to make reliability calculations simpler. [Read more…]

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Reliability Questions for the Drone Industry

Reliability Questions for the Drone Industry

FPV quadcopter by Steve Lodefink
FPV quadcopter by Steve LodefinkIn a few Twitter conversations, I’ve learned about the perceived lack of reliability of commercially available quadcopter or drones.

And, being encouraged to write a paper or two on drone reliability. Now that Amazon has a delivery drone patent, and industrial applications continue to announced daily, there is a need for serious reliability in these devices.

The early adopters and explorers in any nascent industry generally discover the many design faults including reliability issues. That is common.

As the drone industry develops, improving product reliability becomes a business necessity. For industrial application it is essential. [Read more…]

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Reliability Questions to Ask During a Review

Reliability Questions to Ask During a Review

Asking the right question is important.

During a review meeting (informal or formal) asking a few reliability questions may reveal weaknesses, strengths, or uncertainty. The design team has many priorities and reliability is often difficult to estimate, yet knowing what is and isn’t known provides a clear picture of risks for decision makers.

If you are a decision maker and need to ascertain the reliability risks of the current design, then asking a couple of questions may provide just the insights you need. It also conveys that reliability is on your mind and that you want to have answers that are meaningful and well thought out.

[Read more…]

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What to Specify with Suppliers to Achieve Reliability Goals

What to Specify with Suppliers to Achieve Reliability Goals

Selecting a supplier for components or subsystems involves many aspects including the desired reliability performance.

Once selected the ability of the supplier to provide items that meet or exceed the reliability requirements relies on their understanding of the requirements and operational conditions related to the specific item within the system. It also relies on the supplier’s knowledge of their own design and manufacturing processes as it related to the reliability performance. [Read more…]