Does anyone have a good definition of MTBF ?

A

Al Dykes

Guest
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









--

a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.
 
The link below has a very good condensed explanation about MTBF.
http://en.wikipedia.org/wiki/Mean_time_between_failure


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:

http://www.bluemax.net/docs/Motherboards/General_Info/mtbf.html


Jerry G.
=====
 
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.
Al Dykes
Click to expand...
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
http://dictionary.reference.com

When I want a technical term defined I try
http://www.google.com/search?&q=define:MTBF
(Note the colon after **define**.)

sun.com has a pretty good definition.
beta-dyne.com has another.
 
On 18 Mar 2005 21:48:16 -0500, adykes@panix.com (Al Dykes) wrote:

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
Click to expand...
http://www.reed-electronics.com/ecnmag/article/CA508446



Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca
 
"Al Dykes" <adykes@panix.com> wrote in message
news:d1g3tg$96q$1@panix5.panix.com...
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.
Click to expand...
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.
 
"Jerry G." <jerryg50@hotmail.com> wrote in message
news:1111206414.714837.148280@z14g2000cwz.googlegroups.com...
The link below has a very good condensed explanation about MTBF.
http://en.wikipedia.org/wiki/Mean_time_between_failure


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:

http://www.bluemax.net/docs/Motherboards/General_Info/mtbf.html


Jerry G.
=====
Click to expand...
Methinks someone at bluemax needs to learn something about MTBF.
 
On 18 Mar 2005 21:48:16 -0500, adykes@panix.com (Al Dykes) wrote:

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
Click to expand...
http://www.ab.com/harry/mtbf.html
 
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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