Porous air bearings enjoy some important advantages over conventional air
bearing types such as increased load carrying capacity, higher stiffness and
improved damping. However, these types of bearings have yet to find
widespread acceptance due to problems with obtaining materials with
consistent permeability, instability issues relating to the volume of gas trapped
at the bearing surface in the pores, and manufacturing the bearing without
altering the permeability.
Using a series of fine grades of alumina powder to minimise surface pore
volume it has been demonstrated that it is possible to consistently and
reproducibly manufacture porous bearings by injection moulding and slip
casting. The relationship between powder size, processing conditions, porosity,
mechanical properties and fluid flow characteristics were experimentally
determined. The temperature of processing and the green density were found
to be the controlling parameters in the resulting fluid flow properties for a
given powder size,
Test bearings were produced from the range of processing conditions
investigated. It was found that the fine powder size bearings were stable over
the entire range of test conditions irrespective of their initial manufacturing
route. The most important consideration for the bearing performance was the
quality of manufacture. The bearings were found to be sensitive to the flatness
of their working surface and quality of fit in their test holder.
The bearings were compared with published theories for load capacity and
stiffness. A reasonable agreement was found with load carrying capacity once a
correction for surface roughness was incorporated. Stiffness predictions
provided a useful tool for the analysis and prediction of properties such as
optimum values of permeability for a given geometry, if certain allowances are
made
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