Quantifying Moment Stiffness

In this technical page, we cover our methodology for quantifying the moment stiffness, i.e. bending stiffness of the fluid film layer produced by our porous media air bearings. Consider a rigid beam supported on either end and opposite sides by two porous media bearings, as shown in the below diagram:

If we apply a known force F to the beam on either side at prescribed positions, this will produce a moment at either bearing of M = F*L. This then displaces the beam by an angular distance as shown below:

Note that this method makes use of small angle approximation. By measurement of the air bearing fly heights H1 and H2, angular displacement and the applied forces, the moment stiffness can be calculated as follows:

It is also important to note that this calculation only considers the air film stiffness. Given that the air film stiffness produced by porous media bearings is often stiffer than their supporting structure, it is prudent to consider the structural stiffness as well, for which this should be modified by the equations for springs in series.