Search Results for: mtbf

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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by Semion Gengrinovich Leave a Comment

How to Define Proper Product Reliability Goal

Defining a proper product reliability goal is a critical step in ensuring that a product meets customer expectations and performs adequately throughout its intended lifespan. This also involves a careful balance between the required level of reliability and the associated costs and complexities of achieving that reliability.

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by Fred Schenkelberg Leave a Comment

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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by Larry George Leave a Comment

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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by Nancy Regan Leave a Comment

How Often Should You Schedule Preventive Maintenance?

Ever wondered how often you should perform preventive maintenance tasks? In just 31 seconds, learn the key concept of Useful Life and how it determines the intervals for Scheduled Replacement and Scheduled Restoration tasks. And learn that Mean Time Between Failures (MTBF) does not govern how often you do them.

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by nomtbf Leave a Comment

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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by André-Michel Ferrari Leave a Comment

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,

Catastrophic or adverse event occurrence predictions and trends which in essence are the objective of this article.

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…]

by Fred Schenkelberg 13 Comments

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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by Debasmita Mukherjee 1 Comment

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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by Debasmita Mukherjee Leave a Comment

One-sample t-test

A one-sample t-test is a statistical test to determine if the sample mean differs significantly from a known or hypothesized population mean (expected value). It is commonly used when we have a single sample and want to compare its average to an expected value.

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by Nancy Regan Leave a Comment

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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by Fred Schenkelberg Leave a Comment

Why do we use Weibull++ over JMP?

Why do we use ReliaSoft instead of JMP to Identify the Time to Failure?

This is a question someone posted to Quora and the system prompted me to answer it, which I did.

This question is part of the general question around which software tools do you use for specific situations. First, my response to the question. [Read more…]

by Oleg Ivanov Leave a Comment

Lifetime Evaluation vs. Measurement. Part 2.

Guest post by Oleg Ivanov

A result of life testing can be measurement or evaluation of the lifetime.

Measurement of the lifetime requires a lot of testing to failure. The results provide us with the life (time-to-failure) distribution of the product itself. It is long and expensive.

Evaluation of the lifetime does not require as many test samples and these tests can be without failures. It is faster and cheaper [1]. A drawback of the evaluation is that it does not give us the lifetime distribution. The evaluation checks the lower bound of reliability only, and interpretation of the results depends on the method of evaluation (the number of samples, test conditions, and the test time). [Read more…]

by Oleg Ivanov Leave a Comment

Lifetime Evaluation v Measurement

Lifetime: Evaluation vs. Measurement

Guest Post by Oleg Ivanov

How can we tell whether an iron is hot enough? The answer is obvious: We can measure temperature by using a thermocouple and a meter. But, in practice, we lick our finger and touch the iron. Sizzle…. Yes, it’s hot!

We know a priori the boiling temperature of water and we can evaluate the temperature of the iron. This method has a lower cost. [Read more…]

by Larry George 1 Comment

MIL-HDBK-217G (George) Reality Factor

Originally published in the ASQ Reliability Review, Vol. 12, No 3, June 1992

Insert these pages into your copy of MIL-HDBK-217. The boldface text is changed to MIL-HDBK-217E [1], section 5.2, on parts count reliability prediction. The changes explain how to use “Paretos,” proportions of parts failing in the field, to compute a reality factor that makes predicted Paretos match field Paretos. You can use field Paretos to calibrate predictions for new equipment. You probably have field Paretos on related parts used in your other equipment, which is now in the field. Remember, the field determines reliability.

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