The second part of this review of hydrodynamic lubrication stresses the fact that, although shafts, etc., may have large dimensions, the achievable film thickness compared to the main dimension is extremely small. This film thickness therefore governs the operation, design and scale of the process, and any occurrence causing misalignments and/or distortions of the same order of magnitude endangers correct operation. This scale-effect is studied, and methods are cited to reduce damage from this cause to a minimum, primarily by design, but also by achieving the maximum minimum film thickness. It is proved that such an optimum bearing exists in the case of a bearing of given overall dimensions.
The theory underlying hydrodynamics, and hence bearing calculations, is matehmatically complex. It is therefore important that data relating to the existence of an optimum bearing can be used to derive a simple formula which indicates quickly the possibility of achieving full film lubrication in a particular radial bearing. An existing formula is the Vogelpohl's volume law.
Hydrodynamic bearings often develop a dangerous instability, and the phenomenon underlying this instability is discussed. Since the use of pre-loaded bearings gives far more stable operation, methods of designing these bearings are considered. It is shown that, apart from higher stability, these bearings have the advantage of higher positional- and running-accuracy.
In many bearings thermal effects are a matter of concern. Power loss, heat balance and other thermal phenomena in bearings are discussed, and various ways of preventing or curing difficulties are presented.
The article concludes with a brief outline of topics in hydrodynamic lubrication not studied here, but of apparent importance and interest. Information on these is slowly becoming available. Some of these fields are: gas lubrication, lubrication with unconventional fluids e.g. liquid metals, bearings operating in the turbulent regime, elastohydrodynamics, and the inverse hydrodynamic theory.
