Your Reliability Engineering Professional Development Site
All articles listed in reverse chronological order.
Dr. Nelson created a simple experiment to illustrate the effect of meddling with a stable process. (Dr. Deming did something similar)
While the intent of making small changes to a process may be to make improvements, these perturbations generally increase the variation of the results.
To improve a process requires understanding the source of the variation and often controlled experiments to identify process improvements. [Read more…]
For those that sat for the exam last Saturday – how did it go? what surprised you or confused you? Was your preparation adequate?
Cheers,
Fred
Roll the dice.
It is about that simple if any one product will survive to a specific time. Every product has a chance, not a guarantee. The time to failure for each product is a function of the use, stresses, assembly, latent defects or imperfections, and many other variables.
The result is generally unknown. And, we often establish a reliability goal that includes the probability of success. Keep in mind that a probability is only meaningful when defined over a specific duration.
I don’t.
When you say ‘… uses MTBF’? What is it you’re implying? Do they make important business decisions, or assess product designs, or order spares based on using MTBF?
Probably not.
When you use MTBF, what do you use it to accomplish?
And, if so, does it work for you? Does MTBF actually provide a useful metric related to your product’s reliability performance?
In my experience, MTBF and related metrics are great for meeting requirements or fulfilling requests, not much else. They are not useful for decision making. MTBF is next to useless when ordering spares. And, it is so commonly misunderstood that the report values are often simply misleading.
Do you receive request for MTBF from customers or internal teams? What do they use MTBF to accomplish? Check of as done?
Some may claim they use MTBF as a comparison to previous products. Some claim it provides an insights to the expected reliability performance. Some really do not know what do with MTBF so just ignore the value.
When gathering data for a part count prediction (aka Mil Hdbk 217 or similar) do you request MTBF values? Is so, do you also ask about failure mechanisms, derating parameters, or how/what will most likely fail?
Simply taking the MTBF value provided reinforces that notion that ‘everybody uses MTBF’ and does not provide you or your team useful information.
Data sheets, vendor websites, reliability reports, etc. all contain MTBF (sometimes called life, or reliability, yet the most common reported metric is MTBF or something similar).
MTBF is around us, built into tools, and expected. My contention is that even though it is not useful, it is so common, that it is assumed everybody uses MTBF.
Don’t be Everybody.
You will do a better job reporting reliability as couplets of probability of success and duration, rather than MTBF.
Do something that is useful and easy to use. Do not use MTBF. Be better than everybody. Add value to your organization, to your team, to your customers. Help others by your example, to be like you and not like everybody else.
Another variation of the X-bar and R chart, in this case measuring and plotting individual readings instead of a sample average. The range value is obtained from the current reading and a fixed number of previous readings.
This type of control chart is suitable for calibration or testing situations where it is not practical to create subgroups of items for samples. [Read more…]
This element of a reliability requirement answers the questions of where and under what conditions the product should operate.
It includes storage, transportation, and installation conditions too. One way to think of the environment is to consider the weather around the device. Temperature, humidity, preoccupation, etc.
I’m on vacation and this is just a quick post for the week. The top five posts of NoMTBF.com by visits to date.
How to Calculate MTTF is probably popular as folks may be searching for a way to do this calculation. It’s actually very simple, yet this article asks why would you want to calculate MTBF?
Set a Reliability Goal without MTBF is another recent article and may have gather interest given it may seem impassable to set a goal without MTBF. It is possible and actually useful.
Why The Drain the The Bathtub Curve Matters is a guest post by Kirk Grey. He explores one of the many myths around the common bathtub curve and modern products.
What is the purpose of Reliability Predictions is a guest post by Andrew Roland (3 of the top 5 are guest posts…) where he examines the useful use of predictions and where many have gone astray assuming a use.
Where does 0.7eV Come From – well, actually the activation energy that represents a doubling of rate in a chemical reaction with an increase in temperature of about 10°C is 0.7eV – beyond that Kirk explores the ramifications.
Back at my office next week, and home to find a few more record breaking articles to post. Plus, if you’re interesting in writing a post, either a problem with solution, a case study, or common issue with assumptions related to reliability – let’s see how it does with visits and views.
How a customer uses a product matters. It matters in the amount and type of stress your product receives. It determines the life span. Someone that uses the product often isn’t necessarily going to have a short life span, it might be the lack of use that most damages a product.
The s chart replaces the R chart and provides an increase in sensitivity to variation of the spread of the data.
The s-chart works better with 10 or more items per sample in order to obtain the s (standard deviation) estimate. The use of a spreadsheet or calculator expedites the calculation of the sample standard deviation.
Control charts provide an ongoing statistical test to determine if the recent set of readings represents convincing evidence that a process has changed or not from an established stable average.
The test also checks the sample to sample variation to determine if the variation is within the established stable range. A stable process is predictable and a control chart provides the evidence that a process is stable or not.
Some control charts use a sample of items for each measurement. The sample average values tend to be normally distributed allowing straightforward construction and interpretation of the control charts. The center line of a chart is the process average. The control limits are generally set at plus or minus three standard deviations (of the sample means – commonly called the sampling error of the mean) from the grand average.
Mean time between failure, MTBF is a common reliability metric. It is easy to calculate. It is regularly requested and readily offered.
MTBF also does not contain very much information of any use. [Read more…]
We need to understand what a product should do when working to be able to detect when it has failed.
When a function does not perform as expected that is a failure. Being very clear about an item’s function(s) is vital when establishing reliability goals. [Read more…]
Every process should operate stably. Every process may have many measurements available to monitor either various aspects to the final product or the assembly equipment. There may be hundreds of possible items to measure and monitor.
We do not have the resources nor time to apply control chart principles to each possible measurement. Control charts do not directly add value and they have a cost to maintain and interpret. While it may be tempting to add a dozen or so control charts, as the cost increases the value quickly decreases.
Once asked a customer what they wanted concerning product reliability.
She fully understood that some units will fail, that it’s matter of chance. She seemed understanding of the difficulty creating every product such that none would fail.
Then she confided that all that is fine, as long as the product she buys does not fail. [Read more…]
Sure, it would be easy to use MTBF for a system reliability goal. Your organization has regularly used MTBF. Your customers are asking for MTBF. The competition all use MTBF. Even your vendors supply only MTBF.
Yet, you know it’s not the best metric to use. It’s not accurate, it’s not useful, and you rather use something else.
Yeah! [Read more…]