ELASTOHYDROSTATIC LUBRICATION OF THRUST AND JOURNAL BEARINGS

Abstract

Hydrostatic bearings are extensively used in machine tool industries. These type of bearings can be designed to suit the requirements of the machine tools. Conventionally, hydrostatic bearings are designed using the available bearing performance characteristics data. Generally, these data are developed with the assumptions that, both, runner/Journal and bearing pad/shall are absolutely rigid. However, when a bearing supports heavy load, elastic deformation of the bearing pad/shall becomes large i.e. may be of the order of fluid film thickness. Thus, the fluid film profile gets modified and the bearing performance characteristics are affected. Therefore, for a heavily loaded bearing, the assumption that the bearing Pad/shell is absolutely rigid, is not appropriate and hence the results obtained using this assumption are not very accurate. Therefore, it would be proper to analyze hydrostatic thrust and journal bearings including the flexibility of bearing pad/shell. The work undertaken in this thesis was planned with these forethoughts. In recent years, the theory of elastohydrodynamic (EHD) lubrication has received considerable attention and the hydrodynamic slider and journal bearings have been analyzed in detail including bearing flexibility. But the literature available on the elastohydrostatic CEHS) lubrication is rather scant. Few studies which are available in this area are limited to uncompensated thrust bearings only. In the case of Journal bearings, to the best of author's knowledge, no literature is C iv available on the elastohydrostatic analysis. Therefore, there is a need to study the performance characteristics of compensated thrust and Journal bearings taking into account the flexibility of the bearing pad/shell. The present work is aimed to analyze some compensated thrust and Journal bearing configurations taking bearing flexibility into account. The work contained in this study is broadly catagorized into two parts. In the first part, the static and dynamic performance characteristics of compensated hydrostatic thrust bearings are studied. Whereas in the second part, the compensated hydrostatic/hybrid journal bearings have been studied. The elastohydrostatic CEHS7 analysis of compensated bearings involves the simultaneous solutions of Reynolds equation governing the flow of lubricant in the clearance space, using equation for the flow of lubricant through the restrictor as a constraint and the elasticity equations for the deformation of bearing pad/shell. For the computation of bearing deformation field, the bearing pad/shell, in general, is considered a three dimensional elastic structure. Finite element method has been used for solving the lubrication and elasticity equations, Based on the normalization of the governing equations, a nondimensional parameter called deformation coefficient (Cs) has been defined as a function of supply pressure, thickness of bearing pad/shell, film thickness/radial clearance, and modulus of elasticity of the bearing material to account for the flexibility of the bearing pad/shell.