PERFORMANCE OF MULTIRECESS HYDROSTATICIHYBRID JOURNAL BEARING CONSIDERING COMBINED INFLUENCE OF RECESS SHAPE AND NON-NEWTONIAN BEHAVIOUR OF LUBRICANT

Abstract

The multirecess hydrostatic/hybrid journal bearings have found extensive use in engineering applications such as machine tools, test equipments, radio telescope and in the aerospace industry. Their wide acceptability in the engineering applications is mainly owing to their excellent characteristics in respect of high fluid film stiffness, high load carrying capacity at zero and slow speed, long life, smooth operation and large fluid film coefficient. The performance of hydrostatic/hybrid journal bearing systems get affected by a number of design parameters such as number of pockets, type of restrictor, bearing flexibility, shape of recess, non-linear behaviour of lubricants etc.A thorough review of the literature on multirecess hydrostatic/hybrid journal bearing system indicates that the shape of the recess taken in majority of the studies is mainly rectangular. The main reason for this is the ease in manufacturing and relative simplicity of this particular geometric shape of the recess as compared to other shapes of recess. However, with the advent of computer assisted precision manufacturing techniques such as CAM, EDM, the other geometric shapes can also be easily manufactured. The designer can make better use of a proper geometric shape of a recess during the design process so as to obtain improved bearing performance. Lubricants used in the bearings are generally mineral in nature and additives are mixed with them to improve their performance during lubrication process. The behaviour of polymer added mineral oil is no longer Newtonian and there exists a nonlinear relationship between shear stress(?) and shear strain rate(y). In the present work, the performance of a multirecess hydrostatic/hybrid journal bearing system have been obtained using the non-Newtonian lubricant obeying the shear strain power law z = m(yr iii The analysis of a multirecess compensated hydrostatic/hybrid journal bearing system requires the solution of Reynolds equation governing the flow of lubricant in the bearing clearance space together with the equation of flow of lubricant through the restrictor as a constraint along with appropriate boundary conditions. The finite element method employing Galerkin's technique has been used to obtain the solution of relevant governing equations.The four different shapes of the recess undertaken in this study are square, circular, elliptical, and triangular. The bearing performance characteristics of a 4-pocket hydrostatic/hybrid journal bearing have been obtained in terms of maximum pressure (max)' minimum fluid film thickness (hm;n ), bearing flow (Q), attitude angle (0), fluid film stiffness coefficients (S,,,S,2,S2,,L 22), fluid film damping coefficients (CCCCz2) and the stability threshold speed margin(Y1) for the generally used bearing operating and geometric parameters. iv