Your Reliability Engineering Professional Development Site
Find all articles across all article series listed in reverse chronological order.
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…]
The easy answer is very often. Each time you want to know how long a product will operate. The accompanying question on how well the estimate will match actual performance makes the real answer more difficult.
We regularly and intuitively do reliability predictions all the time. When starting a car at the beginning of a trip, we estimate the ability of the vehicle to complete the journey. When we purchase a phone, we expect it to operate for at least two years (your expectations may differ).
During the design process, we may have formal or informal useful life expectations. It is not knowing if our decisions related to the design will fulfill the lifetime expectations that leads to the desire to know how well the resulting system will operate. We also may need to estimate warranty or maintenance costs, thus knowing what is likely to fail becomes important.
In general, knowing how long something will operate without failure provides the feedback we need to create a viable system that meets our business and customer reliability expectations.
In short, we do reliability predictions regularly to gauge is we are making good decisions. [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…]
Failure modes and effect analysis is a tool to identify potential failures and prioritize based on severity, occurrence, and detection. I like to describe FMEA as an organized brainstorm. You probably have some experience with FMEA.
In some industries, there is a high expectation or mandate to do an FMEA study. In some industries FMEA maybe just another tool to consider using during various stages of the product or asset lifecycle.
In my opinion, FMEA should be a part of your project plan when it is likely to add value.
Value in the sense that the organization will receive an adequate benefit based on the investment to conduct the FMEA study. [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…]
HALT (highly accelerated life testing) is a method to reveal product weaknesses. Design prototypes experience the step-stress application of relevant stresses until failures appear.
The intent is to find design or process related weaknesses early in the design process thus providing time to economically address the issue. Using a build-test-fix approach does improve a product’s robustness and reliability.
Being a useful tool, should you conduct HALT on every project? It seems that revealing weaknesses is certainly useful. [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…]
“How do you know so much about our program?” was a question the quality manager asked after reading the assessment report. The assessment took one day with eight interviews.
The reliability that results is going to happen whether or not the team designing the product or production line deliberately use reliability engineering tools or not. The elements of a product or system will respond to the environment and either work or fail.
While working at Hewlett-Packard I had the opportunity to conduct reliability program assessment of about 50 product divisions. One hypothesis related the number of reliability tasks the team actively used would correlate to their warranty expenses.
That worked to a point. [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…]
The process to create a new product has risk. There are safety, technical, schedule and financial risks
The risk of a product failing more often then deemed acceptable by the customer or the business falls in the realm of reliability engineering to address.
We can provide insight on what is expected to failure and when. Working with the entire team we can influence the design and assembly process to minimize the risk of field failures.
Below is a short list the most common risk reduction tools in our arsenal. [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 is an attribute of an item. The item, with its design, assembly, and use, has a finite probability of working over some duration.
We deliberately attempt to create items that meet our and our customer’s expectations concerning that probability and duration. We maintain and operate our equipment with an expectation of successful performance.
We and our customers are let down when a failure occurs. We do not enjoy the benefits of the item’s value. [Read more…]
