How do you talk about reliability?
How do your customers? And, your suppliers?
If they are all using MTBF, there is a pretty good chance one or more parties involved have some misunderstanding of MTBF. It’s common for some to think it is a failure free period. Or they don’t count early life or wear-out type failures when doing the calculation.
A product failure is a failure and they all count to the customer who has lost the functionality they expected.
The last time I ran across someone asking for MTBF I asked them what they meant or really wanted. They wanted no or few failures over two years. That was good enough. So, I asked them if 98% surviving over the two years was good enough. It was.
It was a very short discussion.
We both knew that having less than 2% fail over two years was only 2 out every 100 units would failure over the two-year period. No assumptions. No misunderstanding.
Sure, we would need to define ‘failure,’ yet I maintain that if the product doesn’t meet the customer expectations, doesn’t work, works poorly, whatever, it is a failure.
The customer defines failure whether we agree with them or not.
Full reliability specification
Most reliability treaties define reliability at some point as including four elements.
- Function – what the product is supposed to do.
- Environment – where and under what conditions.
- Probability – how many will survive without failure
- Duration – over what period of time or cycles.
It is really simple.
My reliability specification recommendations are
- Ask what they mean — get a full reliability specification.
- Find out what is a failure for the customer?
- Use a percentage surviving over a specific time frame. Be clear.
And, by all means, as soon as someone mentions MTBF – ask they what they mean by that term.
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