MTBF help please

[ChristineR]

ChristineR the 4000 MTBF for the box is correct. What you seem to be calculating is the reliability. Reliability is an exponent function of time, e.g. what is the chance that the box will not break down in it’s first year, yes?

In which case R = e^(-t/MTBF)

For 1 year there is a 11% chance of surviving, where t = -8760 (hr in one year), and MTBF = 4000 hrs

CurtC is correct the reliability is exponential.

All software (and calculations) that I use, assumes the MTBF, failure rate etc to be steady state, no allowance is made for the bath tub curves, burn in, infant mortality, nor ware out phases.

If the rate is assumed to be steady state, then you are correct. Let me see if I can cook up a simpler example.

A card lasts 4 hours. There are 100 cards.

Hour cards alive after %failure this hour survival rate #cards
1 90 .1/1 .9 10
2 70 .2/.9 .78 20
3 40 .3/.7 .57 30
4 0 .4/.4 0 40

The MTBF is (.1 + .2*2 + .3*3 + .4*4) = 3.0 hours

For two cards in one box:

Hour___ survival rate____boxes alive after__#cards
1______.9 * .9 = .81________81___________19
2______.78 * .78 = .61_______49___________32
3______.57 * .57 = .32_______16___________33
4______0___________________0___________16

The MTBF is (.19 + .32*2 + .33*3 + .16*4) = 2.46 hours

I can't guarantee there's no math errors in here, but hopefully you'll all get the idea.

All this is different if you assume a linear life distribution.

 

[chance]

I’m confused, possibly due to a difference in terminology, but are you adding MTBF’s like 500 + 500 = 1000?, as it should be 250.

Also are you trying to show a MTBF with the bathtub curve included?