## Provision spares with confidence

## Provision spares with confidence

(OP)

Last year I asked a question in https://www.eng-tips.com/viewthread.cfm?qid=437516 about a function to give the number of spares required for a lifetime purchase given only the number of operating hours and failures to date, the operating hours remaining, and the confidence level. -- something I thought would be pretty common, but apparently not. In the end, I wrote a VBA function for Excel to do just that via numerical integration:

= NumSpares(HrsSoFar, nFail, HrsToGo, ConfMin, [Stats])

E.g.,

=NumSpares(100000, 0, 150000, 95%) returns 1

=NumSpares(100000, 0, 150000, 95%, 1) returns

1 98.4% 27,109 3,949,789[/tt]

... which means 1 spare gives 98.4% confidence and shows the MTBF integration limits.

=NumSpares(100000, 0, 150000, 95%, 2) returns

0 88.9% 27,109 3,949,789

1 98.4% 27,109 3,949,789

... which means 0 spares gives 88.9% confidence and the second line is as described above. The output lists all results from 0 spares to the number that achieves the required confidence.

I'm happy to post the code if anyone want to give it a test drive and compare the results to any alternative method.

= NumSpares(HrsSoFar, nFail, HrsToGo, ConfMin, [Stats])

E.g.,

=NumSpares(100000, 0, 150000, 95%) returns 1

=NumSpares(100000, 0, 150000, 95%, 1) returns

1 98.4% 27,109 3,949,789[/tt]

... which means 1 spare gives 98.4% confidence and shows the MTBF integration limits.

=NumSpares(100000, 0, 150000, 95%, 2) returns

0 88.9% 27,109 3,949,789

1 98.4% 27,109 3,949,789

... which means 0 spares gives 88.9% confidence and the second line is as described above. The output lists all results from 0 spares to the number that achieves the required confidence.

I'm happy to post the code if anyone want to give it a test drive and compare the results to any alternative method.

## RE: Provision spares with confidence

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

## RE: Provision spares with confidence

Here's the code:

## CODE

## RE: Provision spares with confidence

3949789 hr MTBF results in exp(-100000/3949789) = 97.5%, which looks like that's because of the two-side choice

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

## RE: Provision spares with confidence

Thanks for looking.

## RE: Provision spares with confidence

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

## RE: Provision spares with confidence

I'd be very interested if you compared and posted results with any other source you have.