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Prep notes for ASQ Certified Reliability Engineer exam ISSN 2165-8633
The idea of the CRE Preparation Notes series is to provide you short practical tutorials on all the elements that make up the ASQ CRE body of knowledge. The articles provide introductionary material, basics, how-to’s, examples, and practical use guidance for the full range of reliability engineering concepts, terms, tools, and practices.
Keep your knowledge fresh with regular review of topics and tools that make up reliability engineering.
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You will find the most recent tutorials in reverse chronological order below. Below each article is the section and specific clause of the CRE Body of Knowledge that tutorial addresses. Click on those tags to find other articles on the same topic. To the right on the sidebar, there is a listing of the 7 major categories in the body of knowledge - it's a quick way to find groups of articles on each specific area. You can also use the search function to locate articles, podcasts, or tutorials on specific topics.
Let’s take a graphical view of reliability improvement that occurs during product development or improvement projects.
If we are making improvements the system reliability should increase. We can use the build, test, fix approach to measure improvements, find failures, design improvements, and repeat. [Read more…]
The root sum squared (RSS) method is a statistical tolerance analysis method.
In many cases, the actual individual part dimensions occur near the center of the tolerance range with very few parts with actual dimensions near the tolerance limits. This, of course, assumes the parts are mostly centered and within the tolerance range.
RSS assumes the normal distribution describes the variation of dimensions. The bell-shaped curve is symmetrical and fully described with two parameters, the mean, μ, and the standard deviation, σ. [Read more…]
To help you with taking standardized exams such as ASQ Certification exams, here are some tips that I learned a long time ago that have helped me. There may be cultural differences between the USA and other countries that would invalidate some of these. If anything that I included goes against what the specific examination authority recommends for test taking rules or strategy, go with their recommendations.
There may be cultural differences between the USA and other countries that would invalidate some of these. If anything that I included goes against what the specific examination authority recommends for test taking rules or strategy, go with their recommendations. [Read more…]
The WordPress.com stats helper monkeys prepared a 2014 annual report for this blog. Thanks to everyone for making this blog and program a success.
Here’s an excerpt:
The concert hall at the Sydney Opera House holds 2,700 people. This blog was viewed about 26,000 times in 2014. If it were a concert at Sydney Opera House, it would take about 10 sold-out performances for that many people to see it.
Products that fail may create an unsafe situation.
For catastrophic failure mechanisms, the design team may consider establishing a safety factor or margin of safety policy. This provides the design team guidance as they size structures, select components, and evaluate performance and reliability.
A safety factor or margin are measures of the separation of the stress and strength for a specific failure mechanism. If something has a 2x safety factor it implies the element is twice as strong as the expected stress. [Read more…]
The short answer is, everything varies.
The longer answer involves the agreement between what is possible and what is desired.
If we could design a product and it could be replicated exactly, including every element of the product, we would not need tolerances. Any part would work with any assembly. We would simply specify the dimensions required. [Read more…]
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.
The worst case method simply adds the dimensions using the extreme values for those dimensions. Thus if a part is specified at 25mm +/- 0.1mm than use either 25.1mm or 24.9mm, whichever leads to the most unfavorable situation. [Read more…]
In the past, a reliability engineer developed their skills over years of practice under a mentor.
Today, a reliability engineer is a jack of all trades and needs to be a master of them as well. Reliability has developed into its own science and is a discipline in its own right which crosses many fields of engineering, statistics, and physics. One way to show you have the broad understanding needed to succeed in the diverse field of reliability is the Certified Reliability Engineering (CRE) from the American Society of Quality (ASQ). [Read more…]
One method to define an organization’s approach to reliability is to create a reliability manual. Many organizations already have a quality manual as required for ISO 9000 or similar certification. Reliability may be a section of the larger quality manual or simply integrated into the same document. [Read more…]
The ASQ CRE exam is difficult. The individual elements of the body of knowledge are not in themselves difficult, it is just such a broad range of topics that mastering all the subjects is a challenge.
The CRE Body of Knowledge includes elements of leadership & management, testing & failure analysis, and basic & advanced & reliability statistics. Reliability engineers work across the spectrum of consumer product design to plant maintenance.
Depending on your experience you may have a wealth of experience with availability modeling and repairable system data analysis, and have little experience with design for reliability practices. [Read more…]
Common mode or common cause failures related to redundant systems where one cause can lead to the failure of otherwise redundant elements leading to system failure.
Elements which should fail independently are under some circumstances dependent.
When considering the probability of individual paths in a complex redundant system, take due care to consider the common mode failures which may have a higher probability than any single path in the system. [Read more…]
The operating characteristic curve is useful to understand the capability of a lot sampling plan. It depicts a graphical relationship between the unknown lot’s defect rate and the probability of the specific sampling plan to accept the lot. Ideally, we want a sampling plan the correctly accepts good lots and rejects bad lots. [Read more…]
We have statistics to describe the variation that occurs in our world. Statistics is the language of variation.
If each of your produced products were identical in every way to all products produced, with no variability, we wouldn’t be concerned with the effect of variation on the performance and reliability of our designs.
Yet, variation does happen and we have a range of tools to identify and minimize the naturally occurring variation. [Read more…]
Fault Tree Analysis (FTA) provides a means to logically and graphically display the paths to failure for a system or component. One way to manage a complex system is to start with a reliability block diagram (RBD). Then create a fault tree for each block in the RBD.
Whether a single block or a top level fault for a system the basic process to create a fault tree follows a basic pattern. [Read more…]
Is a logical, graphical diagram that organizes the possible element failures and combination of failures that lead to the top level fault being studied.
The converse, the success tree analysis, starts with the successful operation of a system, for example, and examines in a logical, graphical manner all the elements and combinations that have to work successfully.
With every product, there are numerous ways it can fail. Some more likely and possible than others. The FTA permits a team to think through and organize the sequences or patterns of faults that have to occur to cause a specific top level fault. The top level fault may be a specific type of failure, say the car will not start. Or it may be focused on a serious safety related failure, such as the starter motor overheats starting a fire. [Read more…]