Articles

Find all articles across all article series listed in reverse chronological order.

Safety Factor

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…]

by Fred Schenkelberg 2 Comments

Why do Tolerance Analysis

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…]

by Fred Schenkelberg 2 Comments

Worst Case Tolerance 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.

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…]

by Fred Schenkelberg 1 Comment

Purpose of Tolerances

The short answer is, everything varies.

The longer answer involves the agreement between what is possible and what is desired.

Instead, variation happens.

Widths, lengths, weights, roughness, hardness, and any measure you deem worth specifying will vary from one part to the next. Manufacturing processes impart some amount of variation between each item produced. In many cases, the variation is acceptable for the intended function. In some cases, the vacation is unacceptably large and leads to failures. When the design does not account for the variation holes will not align, components will not fit, or performance will be poor.

[Read more…]

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Consider Variation for Reliable Designs

The better reliability performing systems start the design process with controlling variability.

Variability of materials and processes involved thought the product lifecycle. Reliability performance occurs as a result of the decisions made throughout the design process.

When focused on understanding and minimizing variability, the design becomes robust and reliable.

[Read more…]

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CRE in Application

Guest post by Daniel C. Conrad

In the past, a reliability engineer developed their skills over years of practice under a mentor.

Today, a reliability engineer is a jack of all trades and needs to be a master of them as well. Reliability has developed into its own science and is a discipline in its own right which crosses many fields of engineering, statistics, and physics. One way to show you have the broad understanding needed to succeed in the diverse field of reliability is the Certified Reliability Engineering (CRE) from the American Society of Quality (ASQ). [Read more…]

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Steps to Improve Supplier Reliability

Situation

It’s Friday afternoon and the phone rings. It is another customer complaining about your product not working. This is the fifth call this afternoon. Something is wrong and you’re responsible for making it right.

The natural failure analysis process starts across your team. Gather information, determine the scope of the issue, work to understand the root causes, and implement an appropriate solution. This may involve stopping production, halting shipments, or even a product recall.

Initially, you just have more irate customer calls than a typical Friday afternoon. [Read more…]

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Elements of a Reliability Manual

One method to define an organization’s approach to reliability is to create a reliability manual. Many organizations already have a quality manual as required for ISO 9000 or similar certification. Reliability may be a section of the larger quality manual or simply integrated into the same document. [Read more…]

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How to Prepare for the CRE Exam

The ASQ CRE exam is difficult. The individual elements of the body of knowledge are not in themselves difficult, it is just such a broad range of topics that mastering all the subjects is a challenge.

The CRE Body of Knowledge includes elements of leadership & management, testing & failure analysis, and basic & advanced & reliability statistics. Reliability engineers work across the spectrum of consumer product design to plant maintenance.

Depending on your experience you may have a wealth of experience with availability modeling and repairable system data analysis, and have little experience with design for reliability practices. [Read more…]

by Fred Schenkelberg 1 Comment

How to Connect Reliability Goals to Business Objectives

Reliability goals provide you and your team a focus for the reliability program. They provide a measurable way to design, test, and maintain systems that meet customer expectations.

A goal of any kind in a business is relatively easy to set and publish. They are not easy to entwine into the culture of the organization so the objectives desired by achieving the goal become a meaningful focus. A product development team may have hundreds of pages of specifications and a long list of priorities and objectives. Simple listing a reliability goal, no matter how clearly stated, may not be sufficient to garner the interest of your team.

Simple listing a reliability goal, no matter how clearly stated, may not be sufficient to garner the interest of your team.

[Read more…]

by Fred Schenkelberg 4 Comments

Common Mode Failures

Common mode or common cause failures related to redundant systems where one cause can lead to the failure of otherwise redundant elements leading to system failure.

Elements which should fail independently are under some circumstances dependent.

When considering the probability of individual paths in a complex redundant system, take due care to consider the common mode failures which may have a higher probability than any single path in the system. [Read more…]

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Reliability Engineering is More Than Tools

Reliability engineering is a blend of disciplines from material science to asset management. We use problem-solving, design, maintenance, and statistical tools on a regular basis, yet that is not the only thing we do.

Having met a few engineers that define their role as a reliability engineer as conducting HALT or FMEA only, strikes me as to what most believe we do, or should do, as a reliability engineer. It is true that someone may specialize by choice or chance on one tool, yet even then is that all they do?

I don’t think so.

[Read more…]

by Fred Schenkelberg 3 Comments

OC Curve with Binomial Method

The operating characteristic curve is useful to understand the capability of a lot sampling plan. It depicts a graphical relationship between the unknown lot’s defect rate and the probability of the specific sampling plan to accept the lot. Ideally, we want a sampling plan the correctly accepts good lots and rejects bad lots. [Read more…]

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The True Importance of Reliability Block Diagrams

A reliability block diagram is a graphical and statistical representation of the reliability structure of a system.

Graphical as an RBD is drawn with blocks for each element of a system and connecting lines representing the relationship between elements.

[Read more…]

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Sources of Variation

We have statistics to describe the variation that occurs in our world. Statistics is the language of variation.

If each of your produced products were identical in every way to all products produced, with no variability, we wouldn’t be concerned with the effect of variation on the performance and reliability of our designs.

Yet, variation does happen and we have a range of tools to identify and minimize the naturally occurring variation. [Read more…]