In some applications sleeve bearings are superior to ball bearings, but within the confines of the computer, the current peripherals are not taking into consideration the extra measures necessary to keep sleeve bearings in top performance. When ball bearing fans fail, they slow down or rattle (after years of abuse mind you) but when sleeve bearings fail (under ideal conditions of a computer system) the failure is more absolute, noise or total seizure impart immediate loss of performance, ball bearings seem to fail more gracefully over time, with plenty of warning signals.

Those two traits are what distinguish ball bearings as superior to sleeve bearings in many fan systems (and in MY opinion). Many but not all. Sleeve bearings require more intensive designs, and long-term planning. Ball bearings tend to be more forgiving. Hence when presented with two fan systems, why is the ball bearing based fan considered the better choice?

Because most 60mm x 10mm fans are made by OEM's in great quantities for very small costs per unit. To be competitive with other manufactures the features built into the fan are kept to a minimum. Designing to a minimum is ideal for ball bearings as they do not require much long term thought. The difference then comes when sleeve bearings are used in the same competitive design framework. The result can be a fan designed for ball bearings, but using sleeve bearings. 

	Take for instance this generic OEM fan which at first glance appears to utilize a sleeve bearings. In actuality it uses a 3mm high ball bearing and a 3mm high sleeve bearing - blurring the lines between what is and isn't ball bearing.
This duality emphasizes the point that some small computer fans are using sleeve bearings in situations designed for ball bearings. This fan illustrates the point perfectly - the thrust washer has been enlarged to enable smaller sized ball bearings to be used.

Why do we believe that the fans are being designed for ball bearings and used with sleeve bearings? Well if you look at high quality fans like Nidec or Rodale, or essentially any brand name, you will notice some differences common to their sleeve bearing designs. The sleeve bearings are isolated and shielded from debris and contaminants by complex designs which also help to preserve the lubricant over time.

In this diagram of a Nidec sleeve bearing assembly take note of the "labyrinth" designed into the fan to prevent the sleeve bearings from drying out and failing.

With less expensive fans coming packaged with the average heatsink, most sleeve bearings are easily visible beneath the fans sticker. Some on the other hand, are camouflaged by bronze thrust washers like the fan below. Telling one from the other can become quite complicated with systems of this nature. While the bronze thrust washer in this instance serves no real bearing duty, the interchangeability of the two types of bearings is an interesting point.

If some precautions are taken even the life span of these 'lower' quality fans can be lengthened. With the majority of the the bearings being accessible from underneath a sticker label, a small dab of lubricant on the top of the apparent sleeve bearing will go a long way to keeping them in excellent condition. The oil will be drawn in by capillary actions and distributed towards the shaft. Sleeve bearings, when properly lubricated can last for a very long time and provide performance on par with ball bearings. A reservoir of grease next to the bearing can also be helpful overtime. If sleeve bearings are improperly lubricated (or run at consistenly high temperatures) they can die relatively quickly (6 - 12 mos).