On Sat, 2 Jan 2010 21:56:19 -0800, "Bill Noble"
wrote:

air bearings are much cheaper and work if you don't need to operate in a
vacuum


Air bearings are certainly much cheaper, but they can have stability
problems and tend to fail in spectacular fashion. I worked on a
project a few years ago where we were considering magnetic bearings
specifically for their ability to compensate for instability and load
imbalance on the fly. SKF demo'ed a high speed spindle for us. The
motor was barely audible as we sat in a conference room and the motor
spun at 30,000 RPM in the middle of the table.

But it was godawful expensive, and we ultimately used conventional
ceramic bearing spindles.

--
Ned Simmons