A google search of "MTBF definition" comes up with "Mean Time Between Failure" but I already knew that, and besides, they have not given me a definition. I _think_ the full definition is something like this:
MTBF is the number of hours by which 50% of the units in the population will fail. MTBF is only defined withing the stated serivce life of the unit.
This may be completely off base. Can someone give me a correct definition.
Thanks
The link below has a very good condensed explanation about MTBF.
MTBF is one of the most mis-understood specifications in the industry. If you have a piced of equipment with a 50,000 hours MTBF, this does not mean that it will have some type of failure at that time. In-fact, it may fail long after, or maybe have a chance to fail before. It does not directly mean that 1/2 the units will fail by the MTBF rating.
MTBF is a complex thing. Each component that makes up a complete unit or system can have an MTBF rating. Then you get in to the facturing of having an average MTBF, to be applied to the final product.
The MTBF is a statistcal probability, and is not concise. Most equipment will be expired, obsolete, or be worn out before its MTBF is reached.
Another very good page about MTBF:
Jerry G. =====
The Wikipedia article Jerry's link to is quite thorough so I can't hope to improve on that.
When I want an ordinary word defined I use
When I want a technical term defined I try
sun.com has a pretty good definition. beta-dyne.com has another.
Oddly it is one of the few jargon pieces that is *exactly* what it says. You are very much off base with the above definition - if 50% failed in, say, 100 hours the mean (i.e. average) time between failures cannot be
100hours .To start off think of it as "how long untill the next one fails"..
Then you need to get into probability - particularly confidence intervals.
Methinks someone at bluemax needs to learn something about MTBF.
Regards,
Boris Mohar
Got Knock? - see: Viatrack Printed Circuit Designs (among other things)
MTBF is the (Total number of unit operating hours) divided by the total number of failures. The term MTBF refers to repairable units, i.e., the units can be repared, put back into service and have operating hours continue to accumulate. MTTF (Mean time to failure) refers to non-repairable units. Otherwise, the math is the same for both definitions.
It is important not to confuse MTBF (MTTF) with "life". For example; a lightbulb may have an average life of 2000 hours, and an MTTF of 100,000 hours. How can this be? Consider the following 2 examples:
We put 5 light bulbs on test. The times to failure are: 2200 2000 1980 3000 1500 for a total unit operating time of 10,680. This yields a (incorrect) MTTF of 10,680/5 = 2136 hours.
We simultaneously put 1000 light bulbs on test for a total test time of
200 hours for EACH light bulb for a total unit test time of 200,000 hours. We observe 2 failures. This yields a MTTF of 200,000/2 = 100,000 hours!What's the difference? In the first test, we allowed each bulb to burn out, thus yielding the average LIFE, not the MTTF. MTTF/MTBF are defined for random failures, not wearout/burnout.
The moral of the story is; when measuring MTBF or MTBF, be sure that the units under test have not reached the end of their useful life. The life vs MTTF/MTBF is less of a problem with electronic components, since many of them do not have "wearout" mechanisms. Notable exceptions are electrolytic capacitors.
Have you noticed the MTBF figures that are advertised for Disk Drives? Typically they're on the order of 500,000 hours. This tranlates to over
57 years of continuous operation! However the bearings will wear out long before this time. Most hard disk failures are wearout failures, not random failures. The 500,000 hour MTBF may well be accurate, but we have to be careful how the parameter is applied.I hope this helps.
I hope this helps
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