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.
Restrictor pressure pR: The
pressure ratio in orifice restrictors is limited at the
lower end by the flow velocity that can only go as
high as the speed of sound. For air pR/pS>0.53. With a
larger pressure difference (pR/pS<0.53)
a choked flow is created whereby the mass flow becomes
independent of pR. Specific constructive measures are needed
to obtain a pressure ratio pR/pS<0.53.
Groove dept of shallow groove: The bearing
calculator is based on a homogenous pressure over the groove length obtained with a relative large groove depth. Shallow grooves
are necessary to prevent bearing instability but require finite
element calculations. For custom bearings to be designed to your