How to use MTTF for Maintenance

[Bach Huss]

Hi all,
I understand that MTTF is a component failure of a big system and the component is replaced. We can use minitab to estimate the MTTF.
1. If the calculated MTTF = 300 hrs, what does it mean in layman's term? (is it 50% of the component will fail when operations reach 300 hrs?)
2. Another question, If a vendor publish it's component MTBF to 500 hrs. Do I change all components that reached 500 hrs? (Assuming the component cost and the replacement cost is negligible).

BAch Huss

[Andrew]

1. MTTF is the time at which some percentage of a population can be expected to fail. The percentage depends on the distribution of failure times. If times to failure are normally distributed, then 50% would be correct. On the other hand, if the distribution is exponential, then the MTTF refers to 63.2%.

2. MTBF is similar to MTTF but applies to repairable components. As above, the MTBF value alone is fairly non-informative. There are more variables involved in determining replacement periodicities than the reliability of the component. In addition to the reliability characteristics (more than the MTTF ot MTBF), you need to know the ratio of preventive to corrective replacement costs. Even if replacement costs are negligible, it may not be cost effective to preventively replace if the consequences of failure are minimal.

[tarik]

In addition to what Andrew said, I recommend reading the following material :

1- The Limitations of Using the MTTF as a Reliability Specification: http://www.reliasoft.com/newsletter/2Q2000/mttf.htm

2- Preventive Maintenance: http://www.weibull.com/SystemRelWeb/preventive_maintenance.htm