Proceedings of the 13 th euspen International Conference – Berlin – May 2013 Nanometer accurate orbit model for analysing the error motion of a porous aerostatic bearing S. Cappa 1 , D. Reynaerts 1 , F. Al-Bender 1 1 KU Leuven, Department of Mechanical Engineering, Belgium Steven.Cappa@mech.kuleuven.be Abstract In this work, a nanometer accurate 2D-orbit model is developed for analysing the influence of the minutest details affecting the radial error motion of a porous aerostatic journal bearing. This allows us to increase the running accuracy of an axis of rotation system, such as an aerostatic spindle or rotary table, to the sub-nanometer level. The orbit model is validated experimentally by measuring the radial error motion of a porous aerostatic rotary table with the use of a special measurement technique. The difference between the outcome of the orbit model and radial error motion of the rotary table under test is only 6 nm. 1 Introduction The axis of rotation error motion (ANSI/ASME B89.3.4M standard), designated as error motion in this work, of a well-designed air bearing system will be mainly determined by the machining accuracy of the bearing surfaces. This is so because the clearances of air bearings should be made as small as possible in order to obtain a high stiffness. Hence, the radial error motion of an aerostatic journal bearing with inherent feedholes can be reduced most effectively by increasing the number of feedholes N f , as shown in [1,2]. The radial error motion of an aerostatic porous journal bearing is analysed in this work as this ideally has an infinite number of feedholes and consequently a very low error motion. 2 Mathematical model of a porous journal bearing An externally pressurized porous gas journal bearing is shown in Fig. 1. Pressurized air at constant pressure p s is fed through the porous material wherein the pressure drops to p’ and then to p in the bearing clearance c. Finally, the air exhausts to the