COMPARATIVE STUDIES OF SOME GAS-LUBRICATED LOBED JOURNAL BEARING CONFIGURATIONS

Abstract

Stability of the lubricant over a wide range of temperature, elimination of lubrication feed system, less contamination or radiation damages and capability of operating at very high speeds arc some of the attractive features offered by gas lubricated bearings. These features have gained the gas bearings increasing popularity in a wide variety of applications.The gas bearings pose many challenging problems both to the practising engineers and analytical tribologists. The gas bearings have two familiar short-comings, namely, the low load capacity and the poor dynamic stability due to self-excited whirl. Fortunately the gas bearings in practical applications are usually subject to the light loads. However, dynamic instability in gas bearings is even a more severe problem than that in oil bearings due to high rotor speeds of gas bearing applications. Towards the analytical treatment of gas lubrication the problems are equally tedius. Even for the steady state analysis, the high nonlincarity of the pressure equation poses severe convergence problems in the iterative solution of the equation. In the dynamical state the nonlinear pressure equation becomes explicitly time dependent due to the presence of 0(ph)/3t term. The evalua-tion of the dynamical response of the journal.requires simultaneous solution of the pressure equation and equations of motion. An alternative approach to the dynamic analysis is similar to that of the oil bearings. The approach involves the evaluation of dynamic coefficients and setting up the stability margins via equations of motion. However, unlike oil bearings, the evaluation of dynamic coefficients in gas bearings is not straight forward as the coefficients depend on the whirling frecuency. The analytical problems outlined above have in fact limited the scope of theoretical research in gas lubrication. An important area of research, which is more or less ignored in the literature, is the analysis of the noncircular gas bearings• The research work of this thesis is concerned with gas-lubricated lobed (noncircular) journal bearings. The lobed bearings fall in the class of anti-whirl bearings, which like pivoted-pad bearings, are known to possess the film induced instability inhibition characteristics. Though pivoted-pad bearings arc dynamically superior to the lobed bearings, these bearings, in comparison to the former type, are simpler from design as well as fabrication view points. With oil lubrication an extensive amount of work on both pivoted-pad and lobed bearings exists in the published literature. However, as seen through a survey of gas bearing literature, a considerable amount of work has been reported on pivoted-pad gas bearings, cspecitlly related to the dynamics of these bearings, little attention has been paid to the lobed gas bearings. In fact, the available literature on lobed gas bearings is so scant that it does not provide any useful information to the engineers from design and application viewpoints: This work has been taken up in view of this paucity in gas bearing lAera'tlAire". The present work is directed towards a systematic study of some lobed bearing forms whose applications are now common with oil lubrication. The configurations taken up for the study are four- lobe, three- lobe lemon-bore and offset-halves bearings of which the last two configurations are the geometric variations of a two-lobe bearing. With lobed bearings, the designers may often be confronted with the task of selecting a bearing out of the many available forms. Further, even with a specific lobed bearing, problems may be faced with the selection of the geometric parametersIr The lobed bearing offer an access to zany geometrie para- , meters which can have a aigr4ficant effect on the bearing performance. With the variation of the geometric parameters, optimum geometry of a particular lobed bearing can be set for a preassigned objective. Thus specifically, this work attempts towards a comparative study of the four lobed bearings and their geometric variations with an aim to recommend to the designers the choice of a lobed bearing form and its geometric parameters.