TourTourvacuum preloaded air bearing
fed through simple orifices
 
 Diameter 2R0 (outer diameter bearing)   10-3 m
 Diameter 2R1 (outer diameter porous ring)   10-3 m
 Diameter 2R2 (inner diameter porous ring)   10-3 m
 Diameter 2R3 (inner diameter collar bearing)   10-3 m
 Diameter 2R4 (outer diameter vacuum preload)   10-3 m
 Diameter 2R5 (inner diameter vacuum preload)   10-3 m
 Film thickness h0   10-6 m
 Groove depth h2   10-6 m
 Ambient pressure pA   106 Pa
 Vacuum level pV   106 Pa
 Supply pressure pS   106 Pa
 Restrictor pressure pR   106 Pa
 
 Flow through collar bearing Mb   10-6 kg/s
 Flow through collar bearing Qb   10-3 liter/min
 Flow through vacuum system Mv   10-6 kg/s
 Flow through vacuum system Qv   10-3 liter/min
 Load capacity collar bearing Fb   N
 Tensile force vacuum preload Fv   N
 External load vacuum preloaded bearing F   N
 Axial bearing stiffness S=-dF/dh   106 N/m
 
 

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 specifications, please email info@tribology-abc.com
 
Pressure distribution large groove depth: h2/h0>40
 

 

 
Pressure distribution shallow groove: h2/h0=4
 
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