The MBTF hours are not meant to imply any single fan will not fail
within 50K hours. It is just a statistical tool to allow estimation of
the expected failure rate within a period of time for a number of
installations. If you have 10 servers with 5 such 50K MTBF fans
inside, you will expect to see on the average, 1 fan failure every 42
days or so.
First, you'd have to be assuming the failure rate was
independant of the number of operating hours which it won't
be. Second you have not calculated for a MTBF of 50K above.
Third if the bathtub curve you propose next holds true, the
majority of failures would be clustered towards the 50K hour
mark rather than dispersed inbetween. Forth I hardly ever
use 50K fans, they're almost always 100K. Fifth you'd be
assuming 24/7 operation which isn't necessarily applicable
to anything but a server, but many servers tend to use two
distinctly different fan setups- either 120mm in the PSU
exhaust or in shorter rackmounts, arrays that "could" be
small diameter fans. I will grant you that in all my
projections about fan lifespan thus far, I have completely
ignored the small diameter fans in rackmounts that run at
very high RPM. On these particular fans, they should be
swapped out every year or two, even more often on a valuable
(function) system.
Although if I didn't misunderstand it, most failures will
cluster towards the beginning and end.
With some gear it's possible, though with good QC and sample
testing this can be mitigated.
But in no case would it be
reasonable nor realistic to expect to see zero or minimum fan failure
for the next 5years or so.
Depends on sample size, it could indeed be reasonable if for
example, any fans making a strange noise or vibration were
immediately discarded. Another real example, a few years
back I bought three cases of a specific NMB fan, a model I'd
not bought previously nor since then so they are known
unique and can be discriminated against other NMBs I've
bought, and none of which have failed yet AFAIK... but quite
a few of the systems they're in have been brought back for
other reasons like spyware/viri removal, OS or hardware
upgrade, etc., plus I have several of the fans here in
systems too. I can't be 100% certain that none have failed,
but there is no evidence of it and I do tend to hear about
sytems... usually spyware but occasionally the ones with
popped caps because I got stung on a bunch of Gigabyte
boards from the P3 era, ended up selling 3 different
versions of their boards at the time, all with the bad caps.
@#$% Tom's Hardware liked them, I should have known not to
trust a quick review.
What I know about fans I know from real world experience
with good, and bad, brands. I've hauled off enough dead PCs
and relubed enough fans to have a really good grasp of which
ones stand the test of time. That doesn't mean I can expect
a 100.00% success rate, but where is there a guarantee of
100% in ANY part you'd buy? I'd be a lot more comfortable
guaranteeing against CPU fan failure than motherboard, PSU,
video card (excluding the fan, or rather, presuming fan
didn't fail).
I can understand when someone doesn't believe it, but not
when someone doesn't try to attain long fan lifespan if
they're only getting 2-3 years as mentioned previously in
the thread because that is a very short span of time even
running 24/7.