Reliability Modeling and Predictions Archives

Building and Using a System Reliability Model

From the simplest to the most complex system, building and using a reliability model permits the entire team to make better decisions.

Understanding and monitoring system reliability involves knowing both:

the reliability of elements within the system,
as well as how the elements relate to each other reliability-wise.

We use system reliability models to identify weak links, and focus resources, to meet our desired reliability goals.

Being able to build the right model to meet your team’s needs best is one of your roles as a reliability professional. [Read more…]

by Fred Schenkelberg 3 Comments

Introduction to Physics of Failure Models

When we understand how something fails, we can create a mathematical model of the effect of stress or load on the time to failure behavior.

The model may take different forms, yet it is the ability to related the conditions surrounding the use of a device to its eventual demise that is essential. The specifics include a molecular level of detail in some cases.

Physics of Failure models focus on the particular relationships between stresses and materials. [Read more…]

by Fred Schenkelberg 1 Comment

Standby Redundancy, Equal Failure Rates, Imperfect Switching

First off, switches are not perfect, so this situation includes the reliability of the switch. Switch reliability may be a factor of storage time, probability of actually working when called on to work, or number of switching cycles. Each switch technology may be slightly different. For this equation, you need the reliability or probability of success given the primary unit has failed. Although we are assume the switch reliability is independent of the primary unit reliability and subsequent failure.
[Read more…]

by Fred Schenkelberg 13 Comments

Standby Redundancy with Equal Failure Rates and Perfect Switching

First off, switches are not perfect, so this situation is hypothetical. Yet, when you are exploring adding standby redundancy and haven’t sorted out the switching mechanism, you may be purely curious about the benefits of the redundancy. [Read more…]

by Fred Schenkelberg Leave a Comment

Reliability Block Diagrams Overview and Value

A reliability block diagram is a graphical and mathematical model of the elements of a system permitting the calculation of system reliability given the reliability of the elements.

The model reflects the reliability performance structure including series, parallel, standby and other arrangements of system elements.

Each block in an RBD represents a component or subsystem of the system. The organization and connecting lines represent the reliability structure of the system and may or may not be representative of the system’s functional block diagram. [Read more…]

by Fred Schenkelberg 1 Comment

Field Industry and Public Failure Data

Beyond understanding what will fail (failure mechanisms) we need to know when a system will fail. Both kinds of information can be found in a variety of sources.

Unlike the stock market where we are regularly warned that past performance does not indicate future performance; in reliability engineering past performance is the best predictor of future performance. [Read more…]

by Fred Schenkelberg Leave a Comment

Types of Failure Data

The other day a student said they are asking for failure dates, not failure rates. That is a marked improvement when requesting information for the many sources of reliability information. It is not sufficient though.

Also ask for the type of failures expected or encountered during the testing or observation. [Read more…]

by Fred Schenkelberg 2 Comments

Sources of Reliability Data

We rely on data to make decisions, to reveal patterns or trends, to learn about our systems and world. Data has many forms and sources. Reliability data may provide what will fail and/or when a device will fail. [Read more…]

by Fred Schenkelberg 2 Comments

Reliability Modeling Options

When this posts I should be just getting home from Nepal (jet lag is no place to write posts). So, this post and the next couple are homework for you. Take a look, work the problem, solve it, then show your work. Comment with why you chose your response and why you didn’t select one of the others.

From question 19 of ASQ CRE 2009 sample exam. [Read more…]

by Fred Schenkelberg 5 Comments

K Out of N

The design of a product includes the arrangement of all of the product elements. When considering the reliability of a system, the arrangement matters. Many systems are arranged serially. This means that with the failure of any one element, the system will not work. See the article on Series Systems for more details. [Read more…]

by Fred Schenkelberg 12 Comments

Exponential Reliability

Down to the last week of preparation for the exam on March 2nd. Good luck to all those signed up for that exam date. Time to focus on preparing your notes, organizing your references and doing a final run through of practice exams. [Read more…]

by Fred Schenkelberg 40 Comments

Reliability with Confidence

There are rare situations when we would like to estimate the reliability at the lower confidence level after estimating the mean from a sample (often a test result). And, to make even more rare of a situation, we know the population standard deviation. [Read more…]

by Fred Schenkelberg 3 Comments

Two Pumps Problem

Let’s say we have two identical pumps share a load in parallel. The failure rate for a pump in this mode of operation is 0.0002 failures per hour. If one pump has to carry the full load alone, that pumps failure rate increases to 0.0009 failures per hour.

What is the reliability of the two pump system over a 168 hour week of operation? [Read more…]

by Fred Schenkelberg 2 Comments

Series Reliability Question

Recently a colleague sent me a published copy of the first CRE exam. Scanning through the document suggests that a few things have changed and many have not changed at all. I often comment my enjoyment of the reliability engineering profession, as it doesn’t change too fast and even I can keep up. [Read more…]

by Fred Schenkelberg 25 Comments

How to Calculate Warranty Failures

Let’s say we have a product that most often fails for one major component. Let’s say a fan (it could be anything, and while I don’t have anything against fans, it’s easy to picture).

Ok, this fan has a data sheet with the classic reliability claim of 50,000 hours MTBF. For those that know about my disdain for MTBF (www.nomtbf.com) rest assured I’m not going to get into it here. The basic approach for estimating the number of failure during any [Read more…]