The hard-disk drive industry must move to a new type of spindle motor bearing if it is to continue to increase areal densities and data rates at the pace the market demands.

Ball-bearing technology used in conventional disk drive motors has limitations; ball bearings are not perfectly round, and both balls and races deform slightly under preload. Any imperfections in roundness or in raceways can cause nonrepeatable runout that keeps tracks written to the disk from being regular. This is important because drive design has run into technological barriers that may limit increases in linear bit density. Thus, future capacity increases may largely come from increasing track density. And ball-bearing motors may soon reach the limit of their ability to work at the track densities that will be required.

Fortunately, there is a solution that will allow hard-drive manufacturers to maintain their rate of performance improvements. In fluid-dynamic bearings, the bearing function is taken over by a layer of lubricant less than one-tenth the thickness of a human hair. The rotor supported by the bearing essentially swims around the shaft. The elimination of metal-to-metal contact in fluid-dynamic bearings eliminates nonrepeatable runout due to surface imperfections. That makes it possible to reduce track spacing and increase the tracks per inch on the disk.

Because fluid bearings have no metal-to-metal friction, they can also handle much higher rotational speeds-enough to carry drive design through several more generations.

Fluid bearings have been used in gyroscopes and high-accuracy machine tools for 50 years. But using them in disk drives has been a greater challenge because of the need to manufacture parts in volume with tolerances in the neighborhood of 1 micron. This challenge prevented fluid-dynamic bearings from being used in high-volume applications until the last few years. Fluid-bearing motors were first introduced in disk drives in 1997, and they have a proven reliability record over three generations of volume production.

Ball bearings used in disk drive motors currently are produced for less than a dollar each, and fluid-dynamic bearings need to be cost-competitive to be used in disk drives. Recently, hard-drive manufacturers have achieved major reductions in fluid-dynamic bearing manufacturing costs. Fluid-dynamic bearings require only one-third as many parts as ball bearings, so they have the potential to be produced less expensively.

With those reductions fully implemented, fluid-dynamic bearings should help hard-drive manufacturers maintain rapid increases in storage capacity and access speed over the next few product generations.

Gunter Heine is Senior Vice President of Seagate Technology's Motor Group (Scotts Valley, Calif.).

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