INFLUENCE OF WEAR ON THE PERFORMANCE OF MULTIRECESS FLUID FILM JOURNAL BEARINGS

Abstract

The purpose of lubrication is to separate two surfaces sliding relative to each other with a film of some material which can be sheared without causing any damage to the sliding surfaces. The process of sliding should occasion as little frictional resistance as possible. It is necessary to arrange wherever possible, that there is a lubricant film of sufficient thickness to ensure that no contact occurs between the sliding surfaces, thus eliminating wear, the lubricant and dimensions of sliding surfaces are chosen to enable this to happen. Over the years, the subject of Tribology has been productively recognized as an important aspect in the design and functioning of all rotating machinery to keep separate two sliding surfaces with the help of lubricant film thickness. The special qualities of hydrostatic bearings frequently afford a simple and convenient solution to bring problems experienced with particular machines. Sometimes the only sure method of achieving the required performance of machine is to use a hydrostatic bearing. The revolutionary changes have taken place in the field of hydrostatic and hybrid journal bearing with increase in its applications in many new areas such as micro and nanotechnology, bio-systems, aerospace etc. and from very slow speed telescope to very high speed turbo machinery. The application of proper tribological practices can protect and enhance the life of rotating machinery, improve efficiency of operations, reduce energy consumption and prevent major breakdowns. The growing demands from industries for higher speed applications and ability of hydrostatic/hybrid journal bearing to support heavy loads have necessitated to study the performance of bearings in detail under more realistic conditions. However, the bearings are used extensively in all rotating machinery to support loads and play crucial role in the functioning of chemical, petrochemical, automotive, nuclear applications, power generation, aerospace turbo machinery and process industries all over the world. Owing to the rapid technological developments, demands on the reliable and better performance of various components, in particular multirecess fluid bearings, have been increased considerably. High reliability and successful functioning of the bearings are prerequisites if complex machines and equipments are to operate satisfactorily. That is why bearing should be designed and developed with more realistic data are subjected to extensive performance analysis to ensure maximum operational reliability in service. In quest of this a significant attention has been paid by the researchers and bearing designers in this regard. Therefore, the bearings are required to be designed with more realistic and accurate data so as to meet the demanding requirement for better performance and high reliability. The fluid film bearings are used because of long life expectancies, low power consumption levels, and versatile dynamic behavior. This resulted in many new developments in the area of fluid-film bearings. As a consequence, the multirecess hybrid journal bearing system, which combines both hydrostatic and hydrodynamic action, have come up as an alternative to self-acting hydrodynamic bearing. The multirecess hybrid journal bearings provide excellent performance such as improved load carrying capacity at low and high speeds, increased minimum fluid-film thickness and large fluid film damping. These features make them more suitable for engineering applications such as high speed turbo-machinery, machine tool spindles, precision grinding spindle, reactor coolant pumps, liquid rocket engine etc. Further hydrostatic/hybrid fluid film bearings are classified on the basis of direction of loading as circular/non-circular and conical journal bearings. Circular fluid film journal bearings usually suffer from fluid induced instability at high speed and support only radial load. Fluid induced instability is a condition that is caused due to rotor interaction with the lubricant fluid in the bearings. It can produce large amplitude, self-excited vibrations capable of damaging machine components. These fluid instabilities are known as whirl and whip phenomenon. A significant improvement in the stability threshold speed margin was obtained by making suitable changes in the geometry of the fluid film journal bearing by the analysis. Subsequently the non-circular (multi-lobe) fluid film journal bearing were developed to overcome the problem of fluid induced instability such as whirl and whip. The simplest configurations of non-circular fluid film journal bearings are available as 2-lobe, 3-lobe and 4-lobe journal bearings. These bearihgs are designed as the arc of the circle with centre points placed on the symmetry line of single lobe. The centre of the each lobe does not coincide with the centre of the bearing profile. In a multi-lobe journal bearing the circle inscribed in the bearing profile viii is tangent to the lobe exactly at the middle point of each lobe. They are usually characterized by their number of lobes, bearing aspect ratio and preload factor or offset factor. Further, circular cylindrical fluid film journal bearings usually carry only radial load and does not have the ability to support load in the axial direction, to overcome this drawback, the conical journal bearing may be considered as a further modification of the cylindrical journal bearings. Multirecess conical journal bearings have two significant advantages over the cylindrical journal bearings and separate thrust bearings. The major advantage of such bearing is their ability to support loads in the axial as well as in the radial direction which replace two bearings with one and clearance is easily adjustable on assembly. For this reason, they may be used to substitute assemblies made up of a journal plus a thrust bearing when the thrust is in one direction. Unlike cylindrical bearings, the radial play is not fixed but depends on axial loading, and hence the single-cone bearings may not be suitable when there is high variations of axial thrust. A basic arrangement of a recessed conical journal bearing contains four recess although five or six recesses would have superior load and stiffness performance. Each recess is supplied from a constant-pressure source by way of a flow control device. In general, the most of the time, the fluid film journal bearings operates under poor static and dynamic performance due to the various possible causes such as excessive contact stresses, misaligned loads, lubricant failure or contamination, weaqn bearing, improper selection of lubricant, improper method of compensation etc. and also, due to various unforeseen causes leading to the bearing failure. Inspite of the significant innovation in the development of bearing and bearing materials; these bearings do fail in practice due to wear and journal misalignment with serious consequences, particularly in large installations such as power plants, rolling mills etc. A thorough review of the literature dealing with multirecess journal bearings reveals that the available studies in the area of circular fluid film journal bearings mainly focus attention on different aspects such as flexibility, non-Newtonian behavior of lubricant, turbulence, fluid inertia, surface roughness, running in wear, influence of wear, influence of misalignment of journal in bearing etc. To the best of author knowledge, till date no comprehensive study has yet been reported which shows the influence of wear and the combined influence of wear and misalignment effects on the ix performance of multirecess circular, non-circular and conical fluid film compensated journal bearing systems. As the performance of a bearing is greatly influenced by the choice of compensating device. It has also been observed no comprehensive study is available which shows the influence of various compensating devices such as capillary, orifice, constant flow valve and membrane restrictors on the non-circular bearings. It has been further observed there is a very very limited information available in the area of multirecess conical hydrostatic/hybrid journal bearing system. Keeping in view the gaps as indicated in the literature review. The work reported in the present thesis has been carried out to accomplish the following objectives: o To carry out the performance analysis of multirecess noncircular hybrid journal bearings for the different bearing configurations such as 2-lobe, 3-lobe and 4-lobe bearings. o To study the influence of wear on the performance characteristics of multilobe multirecess noncircular fluid film journal bearing system compensated with different flow control devices such as membrane, orifice, capillary and constant flow valve restrictors. o To study the combined influence of wear and journal misalignment on the performance characteristics of a noncircular multirecess fluid film journal bearing system. o To evaluate the comparative performance of noncircular hybrid journal bearing vis a vis the circular multirecess hybrid journal bearing and the influence of number of pockets on the performance of multirecess hybrid journal bearing system. o To carry out the performance analysis of multirecess conical hybrid journal bearings. o To study the influence of wear on the performance characteristics of multirecess conical fluid film journal bearing system compensated with different flow control devices such as orifice, capillary and constant flow valve restrictors. The results in the present study have been numerically simulated by using the developed computer program in Fortran 77 and have been presented for the generally used values of bearing operating and geometric parameters Lel = 1.0, Wo = 0.2 — 1.0, 16* = 0.5, S2 = 0.5,1.0, at, = 0.14,0.25, 8 = 0.8,1.0,1.2 for multirecess circular, noncircular and conical journal bearing system in order to highlight the influence of the wear depth parameter (6„, = 0.1— 0.5) along with aligned/misaligned journal parameter (cp,4) = 0.15) on the performance of the bearings. This study make possible to have a reliable information about the static and dynamic performance characteristics of multirecess circular, non-circular and conical hybrid journal bearing system. A theoretical model is used to account and analyze the individual and/or combined influence of wear and journal misalignment on the performance of bearing. The flow of an incompressible, isoviscous and Newtonian lubricant through the clearance space of a journal and bearing is governed by the Reynolds equation. The generalized Reynolds equation governing the flow of lubricant between the bearing surfaces has been used. The influence of wear is studied by considering different values of nondimensional wear depth parameter L. A non-dimensional factor known as offset factor (6) has been defined and used to account for the change in geometry of the circular bearing and semi cone angle(y) in case of multirecess conical journal bearing. A pair of non-dimensional parameters (cV) and (tifi) have been defined and used to account for the journal misalignment effect. In the present work the modified Reynold's equation has been solved by taking the flow of lubricant through restrictor as a constraint, along with relevant boundary conditions. The system. equation corresponding to the Reynold's equation, after adjustment for the continuity of flow through the restrictors becomes non-linear for an orifice and membrane restrictor. In the present work, the non-linear equations are solved using the Newton-Raphson iterative method. The static performance characteristics and dynamic performance characteristics have been computed for the generally used values of bearing operating and geometric parameters. The variation in the bearing performance characteristics are presented and discussed for the various values of wear depth parameter (6w), offset factor (6) and journal misalignment parameters (Cr), The results presented in this study indicates that bearing performance parameters like minimum fluid-film thickness, stiffness coefficient and stability margin are influenced due to the influence of wear, journal misalignment, offset factor and semi cone angle for the chosen bearing configurations. Thus, to generate more realistic bearing characteristics design data, these effects are essential to be considered in the analysis. The results presented in the thesis are expected to be quite useful to the bearing designer