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Prep notes for ASQ Certified Reliability Engineer exam ISSN 2165-8633
The CRE Preparation Notes series provides you with short practical tutorials on all the elements that make up the ASQ CRE body of knowledge. The articles provide introductory material, basics, how-tos, examples, and practical use guidance for the full range of reliability engineering concepts, terms, tools, and practices.
Keep your knowledge fresh by regularly reviewing topics and tools that make up reliability engineering.
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You will find the most recent tutorials in reverse chronological order below.
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…]
When time is short and you just want a rough estimate of the standard deviation, turn to the range rule to quickly estimate the standard deviation value.
The standard deviation is approximately equal to the range of the data divided by 4. That’s it, simple. [Read more…]
There are times when you do not have a computer available and would like to visualize the distribution of a small set of data. With paper and pencil, you can create a representation that is similar to a probability density function plot. [Read more…]
This nonparametric hypothesis test tests the equality of population medians. While not as powerful as the Kruskal-Wallis Test, it is useful for smaller sample sizes, when there are a few outliers or errors in the data as it focuses only on the median value. [Read more…]
One of the considerations when planning maintenance is the timing of the replacement of parts of the system. In some cases it makes sense to wait for the item to fail, sometimes, it makes sense to place it on an interval or set an amount of operating time, and yet in others, we can measure indicators that a replacement is necessary.
In order to make these decisions, we need data.
According to O’Connor and Kleyner (Practical Reliability Engineering, 5th Ed.), we need specific information for each part. The following list outlines the recommended information. [Read more…]
In statistics and reliability, we use distributions to describe time to failure patterns. The four functions commonly used in reliability engineering include
We often use terms like, mean, variance, skewness, and kurtosis to describe distributions (along with shape, scale, and location). The mean is defined as the use of a moment generating function. First though let’s first back up to the concept of center of gravity (cog) from mechanics. [Read more…]
We see the terms ‘reliable’ and ‘reliability’ in daily advertising, business names and in casual conversations on a regular basis. Reliability has meaning and importance in our society. Product and brand reputations are made or broken by their product reliability performance.
Reliability testing and data collection is a messy business.
We rarely receive perfect data where all units involved have a precise time to failure record. Sometimes we do not the precise time to failure information or some of the units are still operating.
The data is censored.
There are a few common types of censoring, each of which has statistical techniques to appropriately account for the unknown elements. [Read more…]
It is possible to use the circuit design or the circuit itself to evaluate electronics. This ranges from checking if the system is wired correctly (the right parts are present and attached properly) to a complex evaluation including the proper functioning of firmware and attached software.
Beyond visual inspection, we need a way to evaluate the microscopic structures with today’s electronic components.
The ability to provide inputs and measure outputs provides one view and in most cases is nondestructive. [Read more…]
There are times when we need to evaluate a product to determine is it assembled correctly or when looking for clues concerning a failure mechanism. Simple visual inspection may be sufficient and there are times when we need more information or detail.
A great first step is the use of an appropriate nondestructive evaluation (NDE) method.
Done correctly, we keep the item unchanged and available for shipment or further inspection.
The first time I changed the oil filter of my car (first car in high school) I smashed my knuckles against a grimy block of metal. More than once. It may have been my lack of experience or improper tools. Or, it may have been a rather poor design.
Watching the Indy 500, especially the pit stops, I quickly realized there has to be something different about those cars that lets them change tires, add oil & fuel, and clean the windshield in less time than it took me to get under the hood and find the oil filter. The cars were designed differently and that is what permitted the difference in service time.
In order to increase availability and minimize the cost of maintenance, we have to deliberately design the system to accommodate the needs of maintenance. [Read more…]
Reliability testing to determine what will fail or when will failures occur is expensive.
Organizations invest in the development of a product and attempt through the design process to create a product that is reliable. [Read more…]
Beyond understanding what will fail (failure mechanisms) we need to know when a system will fail. Both kinds of information can be found in a variety of sources.
Unlike the stock market where we are regularly warned that past performance does not indicate future performance; in reliability engineering past performance is the best predictor of future performance. [Read more…]
The other day a student said they are asking for failure dates, not failure rates. That is a marked improvement when requesting information for the many sources of reliability information. It is not sufficient though.
Also ask for the type of failures expected or encountered during the testing or observation. [Read more…]