"The most common, and cheapest, kind of fan bearing is the "sleeve" type. It's a simple design; the steel shaft just sits in a solid bushing."
"The other kind of bearing is the "ballrace", commonly just called a "ball bearing"" -- think small lazy suzy. You can make ball bearings last a lot longer if you oil them once in a while.
http://www.dansdata.com/fanmaint.htm
tags: cpu, fan, case, computer