Development of enhanced capacity air bearings
To run both smoothly and efficiently, high-speed machines need stable, low friction bearings to support their rotor. Being oil-free, self-acting gas film bearings are often the choice in high speed rotating machinery. Although plain aerostatic journal bearings carry more load and have superior lift characteristics, they suffer from instability problems, and one can add grooves to 'increase that stability. The research describe a mathematical model developed for small eccentricities, to predict the steady state and stability of enhanced capacity air bearings which combine external pressurisation and aerodynamic effects by adding grooves. This theory was than tested in a practical bearing design application. With the aid of a computer program geometry and thus load capacity, stiffness and stability were optimised. A prototype bearing based on a standard aerostatic bearing spindle, was designed, produced and tested. Stiffness, load capacity, pressure distribution and stability were monitored experimentally. The effects of the attitude angle on stability are discussed and it is concluded that the attitude angle is an unreliable predictor of bearing stability. Experimental dynamic stability studies have been carried out and the initial bearing spindle was operated with known amounts of unbalance at speeds up to 105000 rpm, with a gas supply pressure of 6 bar. The spindle vibration amplitude was measured and coherence data and dynamic compliance functions of the bearing were determined. The critical speeds were determined by monitoring the natural frequency amplitudes of the bearing spindle. Having observed some positive dynamic benefits of a prototype hybrid spindle, the work was followed up with a further test on a hybrid spindle with improved bearing geometry. The natural frequencies were observed, and the critical speeds determined. The bearing successfully reached the maximum 120000 rpm speed predicted by the theoretical model for the bearing design, and at all speed the spindle remained stable. The main contribution is in improved stability and increased speed capacity of a hybrid externally pressurised bearing by adding optimised geometry grooves which further enhanced the control of pressure distribution for bearings used in high speed spindle applications.