Ball bearings Vs. Sleeve bearings
What's all the fuss over Ball Bearings vs. Sleeve Bearings? For that matter, what's even the difference?
Let's take a few minutes out of normal hardware reviews and explain what
these essential components of every cooling fan are; their differences, their strengths, and most
importantly their weaknesses. Heck we might even have a
few 'tweaks' to throw in...
Since ball bearings
are the favored type of bearing system we'll begin with them. Essentially
ball bearings consist of
two hardened metal rings, one inside the other, separated by a set
number of perfectly spherical steel balls spaced equilaterly with a bearing cage. The balls
run along concave grooves set into each of the rings. Ball bearings can be
found in everything from the washing machines to your hard drive to the little fans
which keep your computer cool 24X7.
The ball system requires a thin film lubricant
for most high speed applications. What lubrication there is, is for friction
reduction, heat dissipation, corrosion prevention, and long bearing life. While the balls make
contact with the bearing cage, most of the friction
in the system is limited to the tangents of the individual balls as
they rotate around in the race ways. The balls have surface finishes
on the order of 2-3 microinches and the raceways of 6-10 microinches. With such
finely tuned components, proper lubrication leads to quieter performance, less torque and the most
important for our situation - long life.
By their very design, ball bearings do not require, nor
allow lubrication after they have left the manufacturer. So long as the
operating environment is ideal (no water, acceptable operating temperature) they
will last for quite awhile.
Eventually everything fails, and even ball bearings have their limitations.
Ball bearing issues:
It's accepted that ball bearings produce more noise then
sleeve bearings - 1 to 3 dBA according to some sources - but the differences are
rarely noticeable during their typical roles in cooling fans. Noise generated by the fan
blades tends to overcome the sound of the bearings. For example, one fan we tested
rotated at 8,000+RPM and was rated to about 60 dB's of sound. When operating
the fan without blades it was nearly silent. Air moving over the blades was responsible
for 95% of the sound generated by the fan and masked the 5%
generated by the bearings themselves.
Ball bearing based fans fail in essentially two
ways; lubricant and mechanical. Mechanical failures occur when something enters
the raceways, distorts the balls or race way, or impedes them from rotating freely.
When such situations occur, the bearing may 'studder' as it rotates,
seize up, make significant noise or rotate at a reduced speed. Unlike their
counterparts, ball bearings which fail generally continue to rotate, but with increased noise output.
Lubricant based failures generally
the bearing has been subjected to extreme temperatures which
alter the viscosity of the lubricant. While dust can also play a role in the life
span of ball bearings, the variety used in computing applications are generally
protected with a metal, plastic, or rubber gasket that halts the entrance of dust into
the bearing system.
It's up the manufacturer to use a quality lubricant, one which can handle
a wide range of temperatures over long periods of time without breaking down. Even a
lifetime of operation in moderately elevated
temperatures can have effect upon the life of a particular fan
(and its bearings). Take for example two identical heatsinks with ball bearing
fans hooked up to cool two 40Watt thermal loads. With Fan A blowing down onto the
heatsink (impingement airflow) and Fan B exhausting air from the heatsink outwards,
which fan will fail first? Fan A operates in the cooler surrounding air, where Fan B
operates in the warm air from the heatsink. Thus Fan B will be the
first to fail - statistically at least. A higher operating temperature translates into reduced
bearing life expectancy.