Reduction of whirl instability in externally pressurised gas journal bearings
One major limitation in the use of externally pressurized air bearings (EPB) is their susceptibility to vibrations problems. The most destructive vibration that develops in a gas bearing is self-excited whirl, which commences when the damping force in the bearing becomes zero. The main aim of this research was to undertake a theoretical investigation of cylindrical whirl in a balanced vertical axis EPB, and ways of delaying its onset. In order to do this, it was necessary firstly to find the bearing static stiffness and damping coefficients by means of a compressible flow analysis of the gas. To do this, by employing Mathematica, a novel approach used the 'shooting method' in order to solve the governing differential equation (which included functions of a complex variable). The results obtained compared very favourably with previous theoretical published work that had employed finite difference methods. Having found the bearing coefficients, it was possible to solve the bearing dynamic differential equations and thus create stability maps. This procedure was carried out on various bearing arrangements aimed at raising the stability threshold. These included: A series arrangement involving a non-rotating externally pressurized sleeve round the rotor as well as a series arrangement in parallel with the main bearing. The series-parallel configuration resulted in a bearing with a threshold speed and stiffness between the extremes of a simple EPS and a series EPB, all three designs being of the same length. An optimum adjustment of the series-parallel geometry, created threshold speeds three to four times that of a simple version. This in itself was an excellent achievement. Finally, a further increase in threshold frequency, up to seven times that of an equivalent simple EPB, was attained by an additional novel design round the sleeve of a passive eddy current magnetic damper.