Your Reliability Engineering Professional Development Site
An easy method to monitor and control a process average. It is an alternative to the Shewhart control chart.
Pre-control charts work well with stable and slow process drifts or changes. These charts provide a means to monitor a process and act as a guide for process centering.
They are easier to setup, implement, and interpret the Shewhart charts. [Read more…]
Every failure provides information. It provides time to failure, stress strength relationship, process stability and design margin types of information. In every case. Even failures directly related to human error.
A hardware intermittent failure observed by a firmware engineer should not be dismissed. Rather recorded, explored and examined.
A single intermittent failure, or glitch, may indicate nothing other than just a totally random glitch, or a design error that degrades over time causing 50% of units to fail in first three months.
The connection between the specifications or drawings or design requirements and the manufacturing process is the capability of the process to consistently create items within spec.
A ratio of the specification over the spread of measured items provides a means to describe the process capability.
The ratios rely on the standard deviation or spread of the produced items. The index is meaningful only if the process is stable. Thus beyond making sure the measurements have minimal measurement error, check the stability using the appropriate control chart.
In this article we are assuming the measurements are normally distributed, yet knowing that is not always the case, you can calculate capability indices using the actual distribution.
The indices will have similar interpretations yet take care when applying these concepts using other than normal distribution data.
Some days are better than others. We sometimes run into failure when working to create a new product. With a little investigation we suspect the components are not working as expected.
We’ll call the vendor and ask for an explanation. If this is normal production and variability of performance, our product will suffer an higher than expected failure rate. The vendor will assure us with:
Not helpful. [Read more…]
The basic idea of reliability growth is the information learned during testing allows the team to make improvements.
The improvements then reveal themselves in the next round of testing. There are improvements during each test phase as the immediate fixes occur.
Plus some improvements may have longer lead times and be implemented in time for the next round of testing. [Read more…]
As reliability professionals know, products fail. They fail for a wide range of reasons and over a broad span of time. We know it happens.
This doesn’t help when it impacts us directly though. When we purchase a product or service, it should just work. We know the odds, we know better, yet the sting of failure remains.
Customer Service provides a range of services, one of which is helping customers receive the benefit of their purchase. We call customer service to report a failure and expect their help making it right. [Read more…]
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…]
What happens when a product you produce fails? You customer may call and return the product. They may expect you to provide a replacement or refund.
Does it matter if the failure was due to a capacitor or motor that you didn’t design, just purchased?
No.
Does it matter if a supplier’s supplier made an error that directly lead to the failure?
No.
You customer experienced a failure and since the purchase was from you, you are expected to make it right.
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.
There are two basic philosophies when creating a reliability plan for a new product or system.
One is to experiment with prototypes as quickly and often as possible, the build, test, fix, approach. Or, you can research and model detailed aspects of the materials and structures to characterize the strength of a product or system, the analytical approach.
Both methods have obvious applications and not so obvious limitations. [Read more…]
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…]
