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  #11  
Old February 2nd, 2012, 05:13 PM
Kathleen Kathleen is offline
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Default Re: MTTR prediction

Hi Sushant,

You are correct, NSWC-98 and NSWC-07 state that failure rates of mechanical components are not usually described by a constant failure rate distribution because of wear, fatigue and other stress-related failure mechanisms resulting in equipment degradation.

Regarding your question on whether the exponential distribution fairly fits the failure times. Given that all you have are failures in a period of time, the closest distribution that could be used to describe that situation is the exponential distribution. However, you are assuming a constant failure rate, which may not be true.

I hope that this helps.

Thank you,
Kathy
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  #12  
Old February 2nd, 2012, 09:52 PM
Sushant Mohan Dewal Sushant Mohan Dewal is offline
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Default Re: MTTR prediction

Dear Kathleen,

Thanks for the clarification. Does it means that to be on the safe side only MTTF/ MTBF predictions from NSWC can be made.

Regards,

Sushant
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  #13  
Old February 3rd, 2012, 08:22 PM
csasun csasun is offline
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Exclamation Re: MTTR prediction

Hello all!

Interesting post and some of my views are as follows:

NSWC provides models for estimating failure rate - average failure rate. Also, we know that the mechanical parts donot follow exponential distribution. Therefore, it is not prudent to estimate MTTF/MTBF using NSWC models (MTTF would be too optimistic)

On the other hand, since the handbook considers all types of failure modes (eg. chain link: roller wear, breakage, corrosion etc.) in computing failure rate, it is expected to approach constant failure rate - as a system. Therefore, it is important to know the correlation between the NSWC and one's own application (in terms of model parameter values) and then decide the time line duration within which the average failure rate is valid.

Any other views or responses are welcome.
Sasun
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