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Linking Reliability Concepts and Safety Performance

Linking Reliability Concepts and Safety Performance

Definition of Reliability

The concept of Reliability is often misused, misunderstood, and misinterpreted. Reliability in its academic root, is defined as the probability that a system will perform its intended function in a specified mission time and within specific process conditions. So, it is in essence a probability.

The time variable is crucial in calculating the reliability of a system. Reliability is the Probability of Success. And 1 minus the Probability of Failure.

$$ \displaystyle \large R\left(t\right) = 1 – F\left(t\right) \:\:\:\: (where \: F = Failure \: Probability) $$

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5 Reasons Rate of Change is Important

5 Reasons Rate of Change is Important

A simplifying assumption associated with using MTTF or MTBF implies a constant hazard rate. Some assume we’re in the useful life section of the bathtub curve. Others do not understand what assumptions they are making.

Using MTTF or MTBF has many problems and as regular reader here know, we should avoid using these metrics.

By using MTTF or MTBF we also lose information. We are unable to measure or track the rate of change of our equipment or system’s failure rates (hazard rate). The simple average is just an average and does not contain the essential information we need to make decisions.

Let’s explore five different reasons the rate of change of a failure rate is important to measure and track. [Read more…]

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QDD 165: Redux: 5 Aspects of Good Reliability Goals and Requirements

QDD 165: Redux: 5 Aspects of Good Reliability Goals and Requirements

5 Aspects of Good Reliability Goals and Requirements

Good reliability requirements are going to drive our design decisions relating to the concept, the components, the materials, and other stuff. So, the moment to start defining reliability requirements is early in the design process. But, what makes a well-defined reliability requirement? There are five aspects it should cover: do you know what they are? 

We’ll describe what makes a good reliability requirement and examples of common (but not good) requirements.

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How We Think About Reliability

How We Think About Reliability

How We Think About Reliability Is Important

Getting on an airplane we think about the very low probability of failure during the flight duration. This is how we think about reliability.

When buying a car we think about if the vehicle will leave us stranded along a deserted stretch of highway. When buy light bulbs for the hard to reach fixtures we consider paying a bit more to avoid having to drag out the ladder as often.

When we consider reliability as a customer does, we think about the possibility of failure over some duration.

And, we really don’t like it when something fails sooner than expected (or upon installation). [Read more…]

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Fundamentals of Metric Monitoring

Fundamentals of Metric Monitoring

An Accendo Reliability recorded webinar event

Following along the idea stated by Lord Kelvin, “…when you can measure what your are speaking about, and express it in numbers, you know something about it…”, we have organizations measuring reliability performance. Not that measuring something is managing it, those are two different activies. Yet, using various metrics that we track on a regular basis does provide some useful information if done well.

Let’s explore a few ways that I’ve seen reliability metric monitoring. Let’s also examine a simple process to establish, maintain, and end metrics. Measuring something has an expense, so it must also provide value. If the value is not there, or it’s not useful for making decisions, then either improve the metric system or end the metric.

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The Worst Reliability Requirement

The Worst Reliability Requirement

by Mark Powell

Most of us have seen reliability specified using a requirement like the following:

The Zeus 5000 SUV shall have an MTBF of 144,269.5 miles with a 90% confidence.

Some readers may not have seen reliability requirements specified in any other way.  What they have always seen has read something like:  The widget shall have an MTBF of X with a Y% confidence.  This reliability requirement structure is rather ubiquitousin military and aerospace specifications, which along with Mil-HDBK-217, have been major influences in reliability specification practices for decades in many industries.

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Renewal vs. Generalized Renewal Process?

Renewal vs. Generalized Renewal Process?

How to distinguish a renewal process from a “generalized” renewal process? Compare observed monthly returns vs. actuarial returns forecasts using actuarial return rate estimates of TTFF and TBF (Time To First Failure and Time Between Failures). A geophysicist masquerading as an Apple reliability engineer said, “It’s too hard to figure out the probability that a return came from a computer made in an earlier year.”  It’s harder if returns could be second, third, or???

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The Fear of Reliability

The Fear of Reliability

MTBF is a symptom of a bigger problem. It is possibly a lack of interest in reliability. Which I doubt is the case. Or it is a bit of fear of reliability.

Many shy away from the statistics involved. Some simply do not want to know the currently unknown. It could be the fear of potential bad news that the design isn’t reliable enough. Some do not care to know about problems that will require solving.

Whatever the source of the uneasiness, you may know one or more coworkers who would rather not deal with reliability in any direct manner.
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Using Crow-AMSAA Plots in Disaster Predictions

Using Crow-AMSAA Plots in Disaster Predictions

Crow-AMSAA Analysis Overview

Crow-AMSAA Plots have a variety of names, such as Reliability Growth Plots or Duane Plots. The term “Crow” comes from Dr Larry H. Crow, who enhanced James T. Duane’s pioneering launch of this methodology, which was developed in the early 1960s (1). Crow successfully applied the method in the US Army Materials System Analysis Activity (AMSAA). The technique has blossomed into large amounts of new applications in industry such as but not limited to:

  • Making reliability more visible and manageable hence contributing to Reliability Improvement Programs,
  • Monitoring Design Optimization and Quality Performance,

In practical terms, The Crow-AMSAA technique involves plotting, most commonly, cumulative failures vs cumulative time on a log-log scale resulting in straight line plots (2). The line slope value (or Beta value) indicates improving, deteriorating, or constant failure occurrences. Due to the straight-line nature of the plots, future failure forecasts can be estimated. In plain words, based on the current trend, when is the next failure expected to occur? This method handles mixed failure modes, so it is, therefore, suitable for the complex nature of the generating units. [Read more…]

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The Army Memo to Stop Using Mil HDBK 217

The Army Memo to Stop Using Mil HDBK 217

Over 20 years ago the Assistant Secretary of the Army directed the Army to not use MIL HBK 217 in a request for proposals, even for guidance. Exceptions, by waiver only.

217 is still around and routinely called out. That is a lot of waivers.

Why is 217 and other parts count database prediction packages still in use? Let’s explore the memo a bit more, plus ponder what is maintaining the popularity of 217 and ilk.

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Reliability Block Diagram Fundamentals

Reliability Block Diagram Fundamentals

Basic Definitions

  • Reliability: Ability to provide continuous and stable system/component function under defined operating conditions without failure(s).
  • Mean Time Between Failures (MTBF): Average expected time interval between failures. Often a measure of reliability.
    • Failure Rate is the expected no. of failures over a period of time. MTBF is the inverse of failure rate.
  • Availability: Percentage of time a system or component can perform its intended function without failure.   
    • Key aspects involve individual component reliability, system architecture, redundancy, and implementation of control strategies to ensure uninterrupted service. 
    • Availability = Uptime/(Total Operation Time) = MTBF/(MTBF+MTTR), MTTR is a measure of time to repair the system/component
    • For example, 99.999% (5-9’s of availability) -> 5 min downtime in one year
  • Reliability Block Diagram: Reliability Block Diagram (RBD) is a graphical representation used to model reliability in complex systems. Illustrates interconnections between various components or subsystems within a system.
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Preventive Maintenance: What It Is & How to Set the Right Intervals

Preventive Maintenance: What It Is & How to Set the Right Intervals

In this video, I’ll walk you through the essentials of preventive maintenance – what it is and how to correctly identify the intervals for preventive maintenance tasks. We’ll talk about scheduled replacement and scheduled restoration tasks, and I’ll take you along as I perform three real-life examples: changing my home HVAC filter, getting my hair colored, and an oil change for my Subaru Forester. Each task highlights the importance of scheduling based on “age,” regardless of the condition at the time.

I also explain why preventive maintenance intervals should be set based on the useful life and not manufacturer recommendations, mean time between failures (MTBF), or criticality. Understanding these concepts is key to building a strong Reliability Culture in any organization. If you have any questions about preventive maintenance, drop them in the comments. I’m here to help!

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