Failure Rate Archives

by Fred Schenkelberg 5 Comments

The 2 Parameter Weibull Distribution 7 Formulas

This is part of a short series on the common life data distributions.

The Weibull distribution is both popular and useful. It has some nice features and flexibility that support its popularity. This short article focuses on 7 formulas of the Weibull Distribution.

If you want to know more about fitting a set of data to a distribution, well that is in another article.

It has the essential formulas that you may find useful when answering specific questions. Knowing a distribution’s set of parameters does provide, along with the right formulas, a quick means to answer a wide range of reliability related questions. [Read more…]

by Fred Schenkelberg Leave a Comment

How to Calculate Reliability Given 3 Different Distributions

On occasion, we want to estimate the reliability of an item at a specific time.

Maybe we are considering extending the warranty period, for example, and want to know the probability of no failures over one year instead of over the current 3 months.

Or, let’s say you talked to a bearing vendor and have the Weibull parameters and wish to know the reliability value over 2 years.

Whatever specific situation, you have the life distributions parameters. You just need to calculate reliability at a specific time. We can do that and let’s try it with three distributions using their respective reliability functions: exponential, Weibull, and lognormal. [Read more…]

by James Kovacevic Leave a Comment

How Equipment Fails, Understanding the 6 Failure Patterns

Knowing How Equipment Fails Allows Effective Plans to Be Put In Place and Improve Equipment Reliability

Identifying How Equipment Fails

In the 1960s the failure rate of jet aircraft was high even with the extensive maintenance programs that were put in place to prevent the failures. The programs required overhauls, rebuilds and detailed inspections which required the various components to be disassembled. All of these activities were based on an estimated save life of the equipment. [Read more…]

by Fred Schenkelberg 3 Comments

Derating Value

This example is based on a real situation. After a class on design for reliability, a senior manager declared that every component would be fully derated in every product (electronic test & measurement devices). Within a year the design team redesigned all new and existing products, with strict adherence to the derating guidelines provided in the class. A year after the class the product line enjoyed a 50% reduction in warranty claims. They learned about derating and a manager saw the potential value. [Read more…]

by Fred Schenkelberg 1 Comment

Reliability Management Terminology

For Reliability Engineers to converse with one another and with non-technical people in an organization, it is necessary for the language of reliability to be widely understood. These terms form the backbone of a working vocabulary and should be well understood by Certified Reliability Engineers.

Availability: Fraction of time that a system is usable. Steady state Availability = MTBF/(MTBF+MTTR) [Read more…]

by Fred Schenkelberg Leave a Comment

Reliability Predictions

If only we had a crystal ball or another device to predict the future. From the general wondering about the enemies next move, to the soldier hoping their equipment will work. In the corporate boardroom estimating the competitions next move, to the maintenance manager ordering spare parts, we have many uses for knowing the future.

We often look to past performance to provide an indication of the future. Has this mutual fund regularly provided adequate returns? If so, we predict it will going forward. And anyone that has reviewed mutual fund performance also has read or heard the admonishment to not use past performance to estimate future returns. Mutual funds, markets, business and battlefields all change and respond in sometimes unforeseen ways. [Read more…]

by Fred Schenkelberg 6 Comments

Common Formulas

Running through a couple of practice CRE exams recently (yeah, I know I should get out more…) found a few formulas kept coming up in the questions. While it is not a complete list of equation you’ll need for the exam, the following five will help in many of the questions. They seem popular maybe because the relate to key concepts in the body of knowledge, or they are easy to use in question creation. I do not know why. [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 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 5 Comments

Life Testing Question

Hi Fred,

I would take this opportunity to ask the reliability guru about bathtub curve for hardware reliability. I am running 27 units for life test for a million cycles around 555 hours. I have one failure at 300,000 cycles, and the rest of the units are running fine. Would this be classified as an early life failure? Also, how do I make a determination of when the early life failure time interval ends and constant failure rate starts in this example based on failure rate of remaining units? Thanks. [Read more…]

by Fred Schenkelberg 4 Comments

Reading a Datasheet

Reading a datasheet to determine a reliability value may take some investigative work. Whenever I see a fit rate based on failure-free testing, I am curious about how they did the testing and the calculations. [Read more…]