SRC Forum - Message Replies

Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

Original Message:

Posted by: Ernie (erniethurlow@yahoo.com )
Organization:Thermal-Flow Designs, Inc.
Date posted: Sat Sep 14 13:33:43 US/Eastern 2002
Subject: 2 Fans in parallel reliability
Message:
Hi- I have 2 fans operating in parallel. Both have a MTBF of 20000 hours. If I am operating both...(dont consider MTTR) would the total MTBF be 10000hrs. Thank you, Ernie

Reply:

Subject: 2 fans
Reply Posted by: Jean-Marie CLOAREC
Organization: LIGERON SA
Date Posted: Mon Sep 16 5:31:47 US/Eastern 2002
Message:
Exactl. Good answer ! Lambda (2 fans) = Lambda(1 fan) + lambda (the other fan).

Reply:

Subject: Wear-Out Devices
Reply Posted by: Joe Dzekevich (jdzekevich@ieee.org )Raytheon
Date Posted: Mon Sep 16 17:01:07 US/Eastern 2002
Message:
Ernie - what you did is correct IF the fan failure rate is exponential. However, fans are wear-out devices. Where did the 20,000 hours come from? Is it a L10 number? Since fans wear-out, two operating fans will wearout in the same amount of time - the failure rates don't add. For fans, you may end-up with two numbers: a MTBF for any electronic controls and a MTTF for wear-out items like bearings. Life is never simple. :^) Joe

Reply:

Subject: 2 fans
Reply Posted by: Jean-Marie CLOAREC (Jean-Marie.Cloarec@ligeron.com )
Organization: LIGERON SA
Date Posted: Fri Sep 20 5:29:31 US/Eastern 2002
Message:
Agree with you Joe. The formula is true only with exponential assumption for failure rate.