Vacuum preloaded bearings
- Do not require opposed bearing action from two
bearings and so only need a single flat guide surface.
Sometimes magnetic preload is applied as an alternative.
- By adjusting the vacuum and pressure, the optimal
stiffness and flying height can be tuned.
- Adjustability of the film thickness can be used for
ultra precision vertical positioning.
- Even accurate angular adjustment is possible when
applied with a flexure mount. Mechanically preloaded
bearings with ball pivots may suffer from friction in
the pivot point.
Bearing principle: The pressurized part of the
bearing can be considered as a collar bearing. This
collar bearing is loaded internally by the action of
vacuum acting on the inner part of the bearing. The
groove connected to ambient pressure is applied to
reduce the flow through the vacuum system.
It's obviously clear that
the flying height (film thickness) in
aerostatic bearings increases with decreasing bearing
load. Or visa versa, the load capacity decreases with
increased flying height. The "tensile" force in vacuum
preloaded bearings is approximately independent of the
flying height. It follows that equilibrium between the
tensile force and the pressure force can be established
for a specific flying height. With decreased flying
height the pressure force increases resulting in load
capacity whereas with increased flying height the
pressure force decreases resulting in a net pulling action.