Ball bearings Vs. Sleeve bearings

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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...

Ball bearings:

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 occur when 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.