On 12/03/2014 07:40 AM, Marius Liebenberg wrote:
> Andy
> MTBF is obtained by testing a number of units over a period of time at
> accelerated environmental conditions.
MTBF CAN be evaluated this way, and it is a more truthful 
way to do it, but it requires a LOT
of units and long testing on hi-rel systems.  So, the DoD, 
long ago, came up with this
crazy scheme to do it ALL on paper, with not a single actual 
test needing to be performed.
This was all worked out by the Defense Electronic Supply 
Center back in the 1970's,
and I watched in total shock and amazement when the trade 
mags described how this
all was to be done.  DESC no longer exists, and I think this 
whole debacle had something
to do with that.

You look up in a book published by DESC what the reliability 
of a specific resistor, transistor,
capacitor, etc. was, and then add up all of those parts and 
put them into an equation
that gives you a total reliability.  NO mechanical parts 
need APPLY!  Motors, fans, bearings,
disk drives, etc. are totally reliable!  Also, all 
connectors, switches, and wiring are
not prone to failure.

The whole system is TOTAL BALONEY, but it is STILL used in 
many industries, because
it makes such WONDERFUL numbers, even though anybody who 
cares KNOWS it
is all FAKE!  (DoD knows the whole methodology is bogus, and 
has gone back to
more supportable methods of assessing reliability.)

Sorry, rant mode off for now.

Jon

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