What is sleeve bearing?

A kind of bearing containing oil as a friction contact when fan is rotating. Its disadvantage is the contact of the bearing gets rough and dry when the oil is leaking or less after certain period of use. The unsmooth contact results in high temperature, abnormal noise and then causes the cooler totally out of order. The life in use of this kind of fan is about 1/2 or 2/3 of ball bearing fan which is around 20000~30000 hours.