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Musings on Reliability and Maintenance Topics

Short essays and thoughts (musings) on reliability and maintenance engineering topics.


Let me know your reaction and thought, plus any questions.

ISSN 2329-0080

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Reliability and Monte Carlo Determined Tolerances

Reliability and Monte Carlo Determined Tolerances

In the Monte Carlo method, one uses the idea that not all parts have the same dimensions, yet a normal distribution describing the variation of the part dimensions is not assumed.

Although the normal distribution does commonly apply, if the process includes sorting or regular adjustments or if the distribution is either clipped or skewed then the normal distribution may not be the best way to summarize the data.

As with any tolerance setting, getting it right is key for the proper functioning of a product. Monte Carlo method allows you to consider and use the appropriate models for the variations that will exist across your components. [Read more…]

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Reliability and Root Sum Squared Tolerances

Reliability and Root Sum Squared Tolerances

The root sum squared (RSS) method is a statistical tolerance analysis method. In many cases, the actual individual part dimensions fall near the center of the tolerance range with very few parts with actual dimensions near the tolerance limits.

This, of course, assumes the part dimensions are tightly grouped and within the tolerance range.

Setting tolerances well, using the best available data about the part(s) variation, allows creating designs that function well given the expected part variation. This is better for reliable performance. Also, the same method can be applied when the loads and stresses are normally distributed.

Check that assumption with you data first, of course. [Read more…]

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Reliability and Worst Case Analysis

Reliability and Worst Case Analysis

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. Setting tolerances such that the system will function given the expected variation of manufactured components improves that ability of the system to perform reliably.

In the worst-case method, you simply add the dimensions using the extreme values for those dimensions. Thus, if a part is specified at 25 ± 0.1 mm, then use either 25.1 or 24.9 mm, whichever leads to the most unfavorable situation.

The actual range of variation should be the measured values from a stable process. It may be based on vendor claims for process variation, industry standards, or engineering judgment. [Read more…]

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Reliability Questions for the Drone Industry

Reliability Questions for the Drone Industry

FPV quadcopter by Steve Lodefink
FPV quadcopter by Steve LodefinkIn a few Twitter conversations, I’ve learned about the perceived lack of reliability of commercially available quadcopter or drones.

And, being encouraged to write a paper or two on drone reliability. Now that Amazon has a delivery drone patent, and industrial applications continue to announced daily, there is a need for serious reliability in these devices.

The early adopters and explorers in any nascent industry generally discover the many design faults including reliability issues. That is common.

As the drone industry develops, improving product reliability becomes a business necessity. For industrial application it is essential. [Read more…]

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Reliability Questions to Ask During a Review

Reliability Questions to Ask During a Review

Asking the right question is important.

During a review meeting (informal or formal) asking a few reliability questions may reveal weaknesses, strengths, or uncertainty. The design team has many priorities and reliability is often difficult to estimate, yet knowing what is and isn’t known provides a clear picture of risks for decision makers.

If you are a decision maker and need to ascertain the reliability risks of the current design, then asking a couple of questions may provide just the insights you need. It also conveys that reliability is on your mind and that you want to have answers that are meaningful and well thought out.

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What to Specify with Suppliers to Achieve Reliability Goals

What to Specify with Suppliers to Achieve Reliability Goals

Selecting a supplier for components or subsystems involves many aspects including the desired reliability performance.

Once selected the ability of the supplier to provide items that meet or exceed the reliability requirements relies on their understanding of the requirements and operational conditions related to the specific item within the system. It also relies on the supplier’s knowledge of their own design and manufacturing processes as it related to the reliability performance. [Read more…]

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Is this FMEA good?

Is this FMEA good?

It is rare that a third party generated FMEA/FMECA has any value. The development or manufacturing teams and supporting staff should comprise the bulk of the study’s team. Team size for a specific study would include 4 to 10 individuals.

The FMEA/FMECA should provide clear action items that may include:

Conduct research or experiments to understand and quantify uncertainty. This may include exploring how an item responds to specific stress, errors or inputs. Or include experiments to estimate the occurrence rating for a specific potential cause of a failure mode. [Read more…]

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Uncovering Hidden Field Failure Problems

Uncovering Hidden Field Failure Problems

A reliable product does not fail often. Customers expect to a level of reliability performance and failures that occur too early dash their expectation.

The design and development team work to create a robust product. To meet the customer’s reliability expectations. The team may use a range of tools to detect any reliability problem prior to launch.

Yet, customers do uncover problems that surprise us. This may be a problem with how we identify and resolve risks, yet it could also be the development process didn’t look close enough to find the issues. Or, worse, we saw the issue and ignored it. [Read more…]

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5 Steps to Create a Reliability Plan

5 Steps to Create a Reliability Plan

A specific reliability plan may include any number of specific tasks. To build an effective plan you need the knowledge of the individual tools and techniques, plus how they may fit together to create an overall plan to achieve your goals.

Reliability Goals

Of course, this starts with establishing complete reliability goals that include function, environment characterization, probability of success (reliability) and duration.

I recommend setting specific goals for setup/installation (early life), the warranty period and the expected customer use period. [Read more…]

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Purpose of a Reliability Program

Purpose of a Reliability Program

The reliable performance of a system is important. It is important to the customer, to our business and to us.

Very few argue that we should ignore the reliability characteristics of a product. We also deem cost, time to market or feature set as important also. The trouble is we can measure the latter directly every day, where the reliability performance is often difficult to measure.

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Duration and Reliability Goals

Duration and Reliability Goals

Coupled with probability is the duration over which the probability applies.

For example, if we desire 99 of 100 to survive, we must state over which period of time this applies. It is proper to state the couplet of 99% reliable over 1 year.

It is not sufficient to define reliability as ‘5-year product’ as it does not contain the information related to how many are expected to survive the 5 years. Likewise, it is not sufficient to say a product has 5 – 9’s reliability (meaning the probability of failure is less than 0.00001) as it does not contain the duration.

If the product has high reliability for only a few seconds, that does not help us make judgments about the first year of life. [Read more…]

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Probability and Reliability Goals

Probability and Reliability Goals

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.

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Environment Element of Reliability Goal

Environment Element of Reliability Goal

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.

[Read more…]