PERFORMANCE OF EXTERNALLY-PRESSURISED GAS-LUBRICATED POROUS JOURNAL BEARINGS UNDER THE INFLUENCE OF MOLECULAR MEAN FREE PATH INDUCED SLIP

Abstract

A porous bearing, with external pressurization is a more recent development in-gas bearing technology. These bearings incorporate the advantages of both the gas lubri-cation and the Porous bearings. The advantages which gases as lubricant offer over the oil lubricants are cleanliness, less contamination and a much wider'range of operating temperatures. The principal adVantage of porous bearings lies in their production by powder metallurgy techniques, offering accurate controls over dimensional tolerances, surface finish and permeability, and providing a least expensive form of externally.pressurized bearing. Recent theoretical and experimental investigations have revealed the superior stability characteristics of these bearings. The self-acting plain bearing are inherently unstable at zero eccentricity. The stability characteris-tics of self-acting bearing and also of antiwhirl lobed bearings improve with increasing load. The externally pressurized bearings on the other hand, are most stable in the unloaded state and their stability characteristics impair with increasing load. ' The present work is concerned with the slip consi-deration which have as yet remained unexplored in the analysis of externally-pressurized gas porous bearings. The usual analyses of these bearings have disregarded the presence of slip flow at the journal and bearing surfaces. On the basis of existing knowledge, two kinds of slip may v) be thought off in these bearings-the tangential velocity slip and the molecular velocity slip. The concept of tangen-tial velocity slip is associated with the slip flow condi-tion at interface of the porous medium and the clearance space. The molecular slip is associated with the slippage of gas molecules at the sliding surfaces when the clearance spaces are of the order of magnitude of molecular mean free path. In the present work only the latter type of slip condition that is, molecular velocity slip has been consi-dered. Investigation in this thesis are based on theoreti-cal analysis using a modified form of generalized Reynolds equation which takes into account the slip effects. It has been assumed that the porous shell is thin and the gas flow in the shell is dominantly radial—This.offers consi-derable simplification in the flow governing equation in the porous shell. The equation of the dynamical pressure component in the clearance and the porous shell have been-framed using linearized system approaches of the previous works. The finite element method has been employed for the numerical solution of the governing equations and the computation of static and dynamic, performance characteris-tics. The numerical computations in all gas bearing problems are expensive.In the present problem the design variables involved are so large in number that a complete performance analysis of the journal bearing is not only more expensive but also quite cumbersome......