The Elsmar Cove Forum and Site Map The Elsmar Cove Wiki More Free Files The Elsmar Cove Forums Discussion Thread Index Post Attachments Listing Failure Modes Services and Solutions to Problems Elsmar cove Forums Main Page Elsmar Cove Home Page

Go Back   The Elsmar Cove Forum > Manufacturing, Service, and Business Systems Processes > Reliability Analysis - Predictions, Testing and Standards


The Elsmar Cove Forum SideBar!
Monitor the Forum
Monitor New Forum Posts
New Threads Feeds
RSS FeedRSS Feed
Sponsor Link










$ Contributor Forum Access
Courtesy Quick Links

Links that Elsmar Cove visitors will find useful in your quest for knowledge:


Howard's International Quality Services

Atul's Symphony Technologies

Dave Scott's Scott Quality Solutions

Praxiom Research Group


NIST's Engineering Statistics Handbook

IRCA - International Register of Certified Auditors

SAE - Society of Automotive Engineers

Quality Digest Portal

IEST - Institute of Environmental Sciences and Technology

ASQ - American Society for Quality


All the Important Standards and Related Web Sites in the World
Reply
 
Thread Tools Search this Thread Rate Thread Content Display Modes
  #1  
Old 26th May 2007, 06:25 AM
itKiwi itKiwi is offline
Inactive Registered Visitor

Registration Date: May 2007
Location: near Milan, Italy
 
Posts: 1
Thanks Given to Others: 0
Thanked 0 Times in 0 Posts
Karma Power: 11
Karma: 10
itKiwi has less than 100 Karma points so far.
Please Help! The Anomaly of Calculating MTBF from Reliability

I would like some help in practical, non-numeric, terms to explain the following “anomaly”.

We all know that Reliability, A = MTBF /(MTBF + MTTR)

So, the higher(longer) the MTBF and the lower(shorter) the MTTR, the better (more 9's) our Reliability.

At the end of our spreadsheet, we have calculated the reliability of our complex system including all the series and parallel elements, redundant elements, etc. A = 0.9999999, etc. Great.

Now, the client wants to know the MTBF of his system. OK. We rearrange the above formula, and we get,

MTBF = A x MTTR/(1-A)

What's this ? MTBF is directly proportional to MTTR, which means the longer my MTTR, the better my MTBF !!! This is contrary to the original basic concept, that a short MTTR is best.

How do we explain this to a non-numeric client ?

It also raises the question; “Which MTTR do I use in this system calculation ? In practical terms, one would think it ethical to use the longest MTTR, (gives us the lowest Reliability). However, this gives us our best system MTBF.

Any answers or discussions on these two points would be appreciated.

Thanks.
Reply With Quote

Sponsored Links
  #2  
Old 26th May 2007, 12:15 PM
Tim Folkerts's Avatar
Tim Folkerts Tim Folkerts is offline
Forum Moderator

Registration Date: Sep 2003
Location: Kansas, USA
Age: 46
 
Posts: 900
Thanks Given to Others: 27
Thanked 249 Times in 150 Posts
Karma Power: 123
Karma: 3974
Tim Folkerts is appreciated, and has over 1700 Karma points.
Tim Folkerts is appreciated, and has over 1700 Karma points.Tim Folkerts is appreciated, and has over 1700 Karma points.Tim Folkerts is appreciated, and has over 1700 Karma points.Tim Folkerts is appreciated, and has over 1700 Karma points.Tim Folkerts is appreciated, and has over 1700 Karma points.Tim Folkerts is appreciated, and has over 1700 Karma points.Tim Folkerts is appreciated, and has over 1700 Karma points.Tim Folkerts is appreciated, and has over 1700 Karma points.
Default Re: The Anomaly of Calculating MTBF from Reliability

itKiwi,

The variable A is typically called the Availability. Reliability is the probability of working at a particular time under specific circumstances. Availability typically considered a constant (a long-term average of how much of the time the system is available); reliability decreases with time.

The two are obviously related, but not the same. A common function to express reliability is
R = exp(-t/MTTF)
but the actual function to use depends on the situation.

Quote:
MTBF = A x MTTR/(1-A)

What's this ? MTBF is directly proportional to MTTR, which means the longer my MTTR, the better my MTBF !!! This is contrary to the original basic concept, that a short MTTR is best.
Ah, but MTBF is not directly proportional to MTTR. MTBF also depends on A, which in turn depends on MTTR. So when you change, MTTR, you also change A.

With a little algebra, it is easy to show that the equation above reduces to
MTBF = MTFB

In other words, MTBF in independent of MTTR, as would be expected.

If you did want to use this equation to calculate to MTBF, you would need the MTTR of the system. This would be some sort of average of the MTTR for each element combined with the probability of each component actually failing.


To calculate the MTBF, the best answer is probably just to go back to the original numbers. If you know the MTBF for each element and the appropriate series & parallel combinations, you should be able to find the overall MTBF directly.


Tim F
__________________
To wonder is to begin to understand.
Reply With Quote
Thank You to Tim Folkerts for your informative Post and/or Attachment!
Sponsored Links

Reply

Lower Navigation Bar
Go Back   The Elsmar Cove Forum > Manufacturing, Service, and Business Systems Processes > Reliability Analysis - Predictions, Testing and Standards

Bookmarks

Tags
mean time between failures (mtbf), mtbf


Visitors Currently Viewing this Thread: 1 (0 Registered Visitors and 1 Unregistered Guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Forum Search
Display Modes Rate Thread Content
Rate Thread Content:

Posting Settings
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Discussion Threads
Discussion Thread Title Thread Starter Forum Replies Last Post or Poll Vote
Calculating MTBF for an electrical component jag53 Reliability Analysis - Predictions, Testing and Standards 8 24th July 2008 05:41 AM
Calculating a Standard Deviation with respect to MTBF bucky88 Reliability Analysis - Predictions, Testing and Standards 1 3rd November 2007 06:44 AM
Calculating the overall MTTR for an MTBF of 476, given four subsystems jaz732 Reliability Analysis - Predictions, Testing and Standards 1 23rd April 2007 03:18 PM
Graphing MTBF (Mean Time Between Failures) or Reliability Data of a System jag53 Reliability Analysis - Predictions, Testing and Standards 1 31st May 2006 07:52 AM
Calculating Series System Reliability and Reliability for Each Individual Component jag53 Reliability Analysis - Predictions, Testing and Standards 4 17th April 2006 09:20 AM



The time now is 08:24 PM. All times are GMT -4.
The time zone can be changed in your UserCP --> Options.



   

All Y'All Come Back Now, Y' Hear?

Made With A Mac! FreeBSD OS Powered by Apache!
Using php4 Forums provided and maintained by Marc Smith Database by MySQL

FAIR USE and CORRECTNESS NOTICE: This site contains copyrighted material the use of which has not always been specifically authorized by the copyright owner. We are making such material available in our efforts to advance understanding of environmental, political, human rights, economic, democracy, scientific, and social justice issues, etc. We believe herein constitutes a 'fair use' of any such copyrighted material as provided for in section 107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section 107, the material on this site is distributed without profit to those who have expressed a prior interest in receiving the included information for research and educational purposes. For more information go to: http://www.law.cornell.edu/uscode/17/ If you wish to use copyrighted material from this site for purposes of your own that go beyond 'fair use', you must obtain permission from the copyright owner. In addition, I do not guarantee the correctness of the content. The risk of using content from the Elsmar Cove web site and forums remains with the user/visitor.

Responsibility Statement: Each person is responsible for anything they post in the Elsmar Cove forum. Neither I, Marc Timothy Smith, nor any of the forum Moderators, are responsible for the content of posts people make. Liability for post content resides with the poster as does interpretation and/or acceptance and/or use of advice by the reader.

Complaints: If you have a complaint with a post in a forum discussion thread, including Content in general, fighting, flaming, copyright infringement, defamation and/or 'slander', please use the 'Report This Post Report This Post Button button which appears at the top of every post in every thread.

Site courtesy of:
Marc Timothy Smith - Cayman Business Systems, 8466 Lesourdsville-West Chester Road, West Chester, Ohio 45069-1929 - USA
(513) 341-6272

To contact me, click the Google Voice link below, enter Your Name and Your Phone Number and Google will ring your phone and connect you for free!

The Elsmar Cove Web Site is *CopyFree*
no new posts