Question: Our experience is that sleeve bearings in motors last much longer than ball bearings. But nothing lasts forever. Why aren't definite life spans quoted for sleeve bearings like those for ball or roller bearings?

Answer: Because the sleeve bearing doesn't fall in a predictable manner. Only unpredictable conditions such as loss or breakdown of the oil, or contamination, will wear out a sleeve bearing.

Normally, there's no metal-to-metal contact within the bearing, and no deformation of the bearing material. The ball or roller bearing, on the other hand, is a precise assembly subjected to a definite load causing its parts to deform. Such specific conditions support statistical prediction of the bearings' lifespan.

Source: Dayton HVAC Motors and Grainger