Characterization of Porous Graphite Air Bearings

Byron R. Knapp, Brian P. O’Connor, Eric R. Marsh Machine Dynamics Research Lab The Pennsylvania State University, University Park, PA Introduction As the demand for higher precision machine tools and inspection equipment grows, there is a need for more precise bearing systems. The error motions of bearings with mechanical contact are dominated by the geometric and surface finish errors of their components [1]. Air bearings, however, do not have mechanical contact between elements and benefit from an air-film averaging effect. Furthermore, due to an absence of mechanical contact, the air bearing has virtually no friction to generate heat and cause wear. An ideal air bearing would distribute the pressure equally across the entire bearing face to increase load capacity and stiffness. By using bearings with porous media compensation, the pressure distribution is nearly constant across the entire face of the air bearing. In addition to their inherent high stiffness and load carrying capacity, porous media air bearings offer better damping and stability characteristics compared to orifice compensated designs [2, 3]. A substantial amount of work has been done to improve the quality of porous graphite air bearings since the work published by Rasnick et al. [2]. The objective of this research is to develop an empirical stiffness model for porous graphite air bearings and verify this model using experimental modal and finite element analyses. The importance of establishing a model for structural performance is evident with respect to deterministic design.