Given MTTF and Number of Items, how to calculate failing parts with Time?

Question

I am wondering how to make use of MTTF

Here is the situation, I am given an MTTF for an item type x and a certain demand for that item in the next 25 years, say 100 parts that will be in operation starting from now.

I need to make sure that failing parts within the next 25 years get replaced, by storing more parts in inventory equal to the number of parts that will fail according to the MTTF.

So given an MTTF of 10000 Hrs and 100 Parts, how many will fail within 25 years? Using a negative expon distribution?

Answer 1

Assuming the failure is distributed according to a negative exponential distrib then convert mttf from hours to years

After i years, the number of failing parts is

nrFailing = nrParts * (1 - e^(-i/mttfInYears))