The MK7 is a 40mm 12 v rated fan. There are thousands of brands and
models, almost everywhere carries one.
FWIW all 40mm fans have a common problem in that they spin relatively
fast and being so small have very little shaft length and thus the
bearing support has unique issues.
90% of the ones you find are sleeve bushings, They put a tiny bit of
the wrong grease in these (remember they are making these for pennies
and they will use the lowest cost everything), and that grease dries
out (the lighter oils and solvents evaporate) leaving behind basically
a waxy solid that sometimes can even be sticky. You gotta remember how
they make grease, it's the heaviest leftovers from crude oil when they
refine it to make gasoline, diesel, and motor oils. It actually
contains small amount of each of those and with heat, they flow out
and that's what lubricates the bushing. Synthetic grease is totally
different, in that it is generally a single compound, rated for it's
intended usage. This is because it starts as a controlled set of
chemicals where natural grease is coming from crude which containes
millions of chemicals and depending on how well it's refined is how
much extra crap there is in it.
So there lies the first rub, that a bushing requires lubrication and
for long term lubrication of it they use grease as the heat makes it
leach out lighter oil in a flow into the bronze (even heat from just
the initial friction in the bearing is enough). Because it's so small,
a drop of oil isn't going to last long. They don't use seals like a
car wheel bearing due to cost and complexity at that size. So again,
99 percent of any fan that small is going to have a sleeve bushing and
they all are going to fail becuase very few companies properly grease
it.
The next rub is upgrade fans often brag about ball bearings but
because a 40mm fan is so small, there typically is not room for 2 ball
bearings. WIth a ball bearing, the load should be directly through the
center of the beaaring in the vertical plane and most don't like an
off axis load. In other words, you never want to take a load like the
fan blades that hangs off the front of the shaft with a single ball
bearing and expect it to last. It will cause the balls inside to skate
(stand still and slide rather than roll) and that leaves flat spots on
the tiny balls making the bearing noisy because internally it's like a
road full of potholes.)
Some fan manufacturers saw the above situtation for single bearing
fans and added a thin bushing at the back to take up the off axis
load. Well, that bushing has even less space, less of the wrong grease
and generally doesn't even last as long as a fan with just a sleeve
bearing before making noise. In other words, the upgrade cheap ball
bearing fans is worse than a good sleeve bushing fan.
The best idea in theory is a dual ball bearing fan, but in a 40mm,
these are hard to find. Further, than can be poor implementation of
said dual ball bearing fan. There is an odd situation where a ball
bearing run at high speed with very little load and no off axis load
can also skate. The very lubricant can cause this. So, good fans put
pre-load ( in other words, the system fixed pressure on the bearing as
load even before the fan blade it added) via a small spring that
attempts to pull the two bearings twards each other but stopped by a
shoulder inside the bearing pocket.
Also need to touch on another type but explain what it is. There are
hydro wave or a hundred other variations but they are all just
bushings. They spiral a groove or other shape into the bushign that
pulls the lubricant (grease) throught the bushing while the shaft pins
to ensure the entire length is lubricated front to back. This
recirculating is not a a bad idea and could improve fan life but as I
explained earlier, the biggest problem is the lubricant itself and the
lack of seals which nobody uses on all the fans I have ever seen of
this size. Cermaic bearings just mean the shaft is super hard compared
to metal and they aren't bearings, they are bushings so don't be
fooled. It is normally reserved for higher end fans and is higher cost
so if combined with a hydro wave bushing and the right grease might be
be a very good fan, but you won't know that from just reading the
typical advertisement at a stores website.
So the truth is, you could buy any number of fans in any number of
price ranges or even recommended by someone but that doesn't mean it's
a good fan. Because even worse than the specs being vague, is that
these are so cheap, you may not even get a fan that matches the specs.
The only way to know is buy an expensive fan from some place like an
electronics supplier like Digikey, Newark, Mouser, etc. I find the
worst offenders are the PC market so places like Newegg have cheap
fans, and of course specs all over the place but hit or miss if it's
actually a good fan.
So rather than fight all that and maybe still not get a good fan, I
just use cheap fans and take them apart with a jewelers screwdriver
and lubricate them myself after cleaning out the crap grease with
rubbing alcohol, with this
http://www.homedepot.com/buy/super-lube-synthetic-grease-with-syncolon-ptfe-3-oz-tube---per-each-21030.html
. Put some on both the shaft and the inside the bushing, then attach
the c clip.
Here's the fans I use
http://ultimachine.com/content/40mm-fan-575-cfm
adn then go back and lubricate with proper synthetic grease. Or, just
lubricate the fan you have.