A STUDY OF SLOT ENTRY HYBRID JOURNAL BEARING SYSTEM CONSIDERING SURFACE ROUGHNESS

Abstract

The slot-entry hydrostatic/hybrid journal bearings are extensively used now-a-days in machine tool industries due to their improved load carrying capacity over recessed bearings. The slot-entry journal bearings has following features, namely commercial availability, improved hydrostatic performance over recessed bearings, good hydrodynamic and hybrid performance, lack of directional variations in stiffness, large bearing lands in the event of sit-down operation and there is no need for external restrictors and non-return valves. A review of the literature concerning both slot-entry bearing and surface roughness effects in bearings has been presented. It has been observed that in most of the studies concerning hydrostatic/hybrid slot-entry journal bearings, the journal and bearing surfaces are assumed to be smooth. However, in actual practice on any finished surfaces imperfections are to be there and these take the form of hills and valleys. The inclusion of surface roughness effects in the analysis of hydrodynamic journal bearings has revealed that it alters the performance characteristics of hydrodynamic journal bearings considerably. Therefore the present work includes the study of performance characteristics of slot-entry journal bearings including surface roughness effects. For computing the bearing performance characteristics, the average Reynolds equation governing the flow of lubricant flow field in the clearance space of a rough surface journal bearing with the equation of flow through the slot restrictor as constraint is used along with appropriate boundary conditions. The finite element method and Galerkin's technique has been used to derive the system equation for the flow field. The Th non-dimensional parameters A (Surface roughness parameter) and y (surface pattern parameter) have been used to represent the magnitude of height distribution of surface irregularities and their orientation respectively. The performance characteristics have been computed for symmetric and asymmetric slot-entry bearing configurations for the various values of surface roughness parameter (A), surface pattern parameter (y) and concentric design pressure ratio ((3*).The stability threshold speed margin in roughened bearing enhances by 30% for symmetric configuration while that of asymmetric configuration enhances by 24% as compared to similar bearing having smooth surfaces. The largest decrement in direct fluid film stiffness coefficient (S,x) is 55.48% in roughened bearing for asymmetric configuration as compared to similar bearing having smooth surfaces. The computed results indicate that the inclusion of surface roughness effects in the analysis alters the performance of slot-entry journal bearing quite significantly vis-a-vis smooth surface bearing. IV