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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

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Original Message:

Posted by: Wayne
Date posted: Fri Nov 29 10:37:21 US/Eastern 2002
Subject: MTBF or service life??
Message:
I often see supplier's specifications on things like for example a motor having an MTBF of 150,000hrs with a service life of 10 yrs. The MTBF greatly exceeds its service life. Is that possible? I thought that MTBF should be lower than service to make sense? So which figure do i use as my input for my reliability analysis of a complete system? MTBF or service life?


Reply:

Subject: Service Life
Reply Posted by: Joe Dzekevich (jdzekevich@ieee.org )
Organization: Raytheon RAL
Date Posted: Mon Dec 2 16:34:50 US/Eastern 2002
Message:
It will be interesting to see what other folks say. I read it as the motor has two failure rates - exponential and wearout. If I take 1/150K x 8760, that gives me a failure rate (returns rate) of 5.8% per year. I would then expect a 5.8% failure rate per year over a 10 year life. Now, they may have meant something else entirely.


Reply:

Subject: MTBF or Service Life?
Reply Posted by: Ken
Date Posted: Wed Dec 4 8:41:22 US/Eastern 2002
Message:
Use MTBF in your analysis. Service life includes requirements for spares availability, depot support, etc., i.e., the item can be supported for the 10 year or other specified period when it does fail.


Reply:

Subject: MTBF or service life?
Reply Posted by: C. Jayaraman Raytheon
Date Posted: Fri Dec 6 13:06:34 US/Eastern 2002
Message:
Use MTBF is calculating system estimates. When MTBF is higher than service life (with repairs, normally) then there is a high probability that the products will survive the service life desired. Again, motor predictions hinge on winding failures and bearing failures (which are weibull). Unless the manufacturer can substantiate the data with real test data I will be inclined to look at the MTBF numbers with a bit of doubt. In looking at service life, do not assume continuous duty, keep the duty cycle in mind. For example, cameras are warranted for 5 years, but is really based on expected number of exposures that a normal user will make in 5 years.


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