Study of Thermal Effects in Non Recessed Hybrid Journal Bearing with Non Newtonian Lubricant

Abstract

The revolutionary changes have taken place in the field of hydrostatic and hybrid journal bearing from very slow speed radar to very high speed turbo machinery. These find application in ultra precision machine tools requiring high stiffness to improve accuracy. The growing demands from industries for higher speed applications and ability of hybrid journal bearing to support heavy loads have necessitated to study the performance of bearings in detail under more realistic conditions. However, the high speed operation of bearings results in higher operating temperature of both lubricant and bearing surface. Increase in bearing temperature poses a danger to bearing liner as well as to lubricant. It not only degrade the performance of a bearing but also of machine itself. Also, an increase in lubricant temperature reduces its viscosity and thereby changes the fluid-film profile. Thus, analysis based on isothermal assumption predicts inaccurate value of minimum fluid-film thickness and increase the risk of bearing failure. The commonly used lubricants for bearing are mineral in nature and additives are added to enhance bearing performance. Generally used additives are high molecular weight polymers and used to control viscosity variation. The behavior of polymer added mineral oil is no longer Newtonian due to non-linear relationship between shear stress and shear strain rate. The most of the studies concerning analysis and design of hydrostatic/ hybrid journal bearings have been carried out assuming the behavior of lubricants to be Newtonian. There is lack of information concerning the non-linear behavior of viscosity of the lubricant on the performance of non-recessed hybrid journal bearings. A thorough scan of the available literature concerning the hydrodynamic and hydrostatic/ hybrid journal bearings indicates that the thermal effects together with non-Newtonian behavior of lubricant due to additives mixed in the lubricants have been ignored in the analysis to obviate the mathematical complexity.