Every computer, laptop or desktop, has a number of fans and motors inside. These fans and motors wear out as they run, and eventually fail. Since motors are a mechanical and spinning component, they must have a bearing to allow the shaft to spin. There are three types of bearings found in computers: sleeve bearing, ball bearing and fluid dynamic bearings.
Sleeve bearings are the least expensive and simplest bearing. They consists of brass bushings, so they run with metal on metal contact. Sleeve bearings are frequently found in small fans, usually 80mm or less. They are frequently used inside power supplies, case fans and small CPU fans (source: Diversified Bronze & Manufacturing Inc.).
Most sleeve bearing fans have a lifespan limited to around 10,000 hours or less. For a typical desktop computer used 8 hours a day, this can provide up to 5 years of usage. But for a computer run continuously, a sleeve bearing motor will fail within a year. As sleeve bearings wear out, the fan slows down and heats up, eventually seizing.
The next better type of bearing is a ball bearing. This bearing features tiny stainless steel balls that reduce friction. Good quality ball bearing fans can last 20,000 hours or more. All hard drives up to 20gb, and some 40gb and 80gb hard drives use ball bearing motors. Hard drives with ball bearing motors will begin to squeal or whine as they wear out. This sound is noticeable after 20,000 power-on hours, and can be very loud after 40,000 hours.
The best bearing type is the fluid-dynamic bearing (FDB.) The FDB is similar to a sleeve bearing but machined to a much higher tolerance and includes a liquid sealed inside the bearing. There are no ball bearings inside an FDB. These bearings are incredibly reliable, operating silently past 50,000 hours. The reduced friction of the fluid dynamic bearing also eliminates heat, making the hard drive and motor run cooler.
Some 80gb hard drives use FDB bearings, and almost all hard drives that are larger than 80gb or larger and operate above 5400-rpm use FDB bearings. FDB bearings can also be found on the very best CPU and case fans, but these require special order since most manufacturers do not include them in the original equipment.
Arctic Cooling sells a line of replacement fans in three standard sizes; 80mm, 92mm and 120mm that include fluid dynamic bearings and feature a 6-year warranty.