PERFORMANCE OF MULTILOBE NON-RECESSED HYDROSTATIC/HYBRID JOURNAL BEARINGS

Abstract

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 bearings are the key machine elements which are used extensively in all rotating machinery to support loads and permit wanted relative motion and restrain unwanted motion. The role of fluid film bearings is crucial in deciding the rotordynamic soundness. Thus, their careful selection and application is a crucial step in the development of predominant rotor-bearing systems. In recent years, researchers are focusing their research efforts on design of hydrostatic/hybrid journal bearing system in order to meet the requirements of modern day machinery. High reliability and successful functioning of the bearings are prerequisites modern day machinery and equipments. Thus, the bearing elements need to be designed more accurately and on the basis of more realistic design data. The high speed and precision required in the machine equipments has led to revolution in the design and developments of various bearing configurations. Therefore, the hybrid journal bearings have been developed and used successfully in machines, which operates under high speed and heavy loading conditions. The excellent dynamic behavior and more run time to failure have enormously increased the application of these bearings. In recent times, the hydrostaic/hybrid bearings have found employed in various engineering applications such as high speed turbo machinery, machine tool spindles, precision grinding spindles, reactor coolant pumps, ultracentrifuges, high speed dental tools, liquid rocket engines, test equipments etc.Circular hybrid journal bearings usually experience fluid induced instability at high speed. Fluid induced instability is a condition that is caused by rotor interaction with the surrounding fluid. It can produce large amplitude, self-excited vibrations capable of damaging many machine components. These fluid instabilities are known as whirl and whip. A significant improvement in the instability threshold speed margin was obtained by changes in the geometry of the fluid film journal bearing. Its suitability mainly stems from the viewpoint of excellent dynamic performance and better stability over circular bearings. Therefore, to suppress and overcome the drawback of fluid induced instability, the multilobe fluid film journal bearing have been developed. In recent times, many researchers have focused their vi research efforts on multilobe journal bearing configurations due to their intrinsic distinctiveness over circular journal bearings. The multilobe bearings are extensively used in high speed application in industry, as they are simple, efficient and economically viable and provide improved stability. In multilobe fluid film journal bearings, the two lobe and three lobe journal bearings are the most commonly used configurations. They are usually characterized by their number of lobes, bearing aspect ratio and offset factor. Despite the fact that, extensive development in lubrication engineering, it was observed that the bearings do fail due to wear in a number of instances. Although the focus of wear related studies are mostly pointed towards improvement and the change of nature of metals. However, the wear defect in bearing due to transient operations (start/stops) is inevitable and thereby appreciably changing the static and dynamic performance characteristics of bearing. Throughout these transitory periods((start/stops), the bearing bush gradually gets worn out owing to abrasive action. As a result of this, the bearing geometry is changed. The lubricant characteristics play a vital role in the operation of a journal bearing system and affect the performance of bearing. Generally, the fluid film journal bearings are designed on the assumption that the lubricant behavior is Newtonian. In modern commercial lubricants, polymer additives are added in order to enhance their performance. The addition of these additives causes commercial lubricants to behave as non-Newtonian fluids. These lubricants do not obey the Newtonian postulate which assumes a linear relationship between shear stress and rate of shear and the analysis of bearing operating with these lubricant, does not give a factual performance of the bearing. Further, the bearing and journal surfaces are generally assumed to be perfectly smooth. In actual practice, the geometric shape of any engineering surfaces is controlled by the characteristics of the finishing processes employed to produce them. Furthermore, the surface roughness heights are typically of the same order as that of the fluid film thickness of journal bearing and this surface roughness alter the fluid film thickness profile and consequently, the performance characteristics of bearing. Bearings are quite often made to operate under heavy loads and higher operating speeds. As a consequence of this, heat is generated within the oil film due to large shearing rates in the lubricant film which increases the temperature of the lubricant fluid-film and the bearing surface. An excessive temperature rise constitutes minimum fluid film thickness and increase in the dissipated power in the lubricant. An increase in speed and load causes reduction in lubricant viscosity and thereby significantly affecting the bearing performance. Thus, the flow field of the lubricant becomes distorted and bearing performance is deteriorated. This may also lead to failure of the journal bearings especially when decrease in minimum fluid vii film thickness occurred. Thus, the prediction of temperature distribution in journal bearings for realistic bearing performance data generation has been the goal of many researchers in the last four decades. The stability of rotating system is an integral facet within the analysis and design of the fluid film bearing systems and plays important role to improve the performance of journal bearing in high speed conditions to enhance the functional quality of the rotating machinery. As a result of this, extensive investigations have been carried out to improve the stability of rotating system After an extensive literature survey of fluid film journal bearing systems, it is found that the studies which deal with multilobe bearing configuration are mainly concerned with multilobe hydrodynamic journal bearing systems. Further, a thorough scan of the available literature in the area of hydrostatic/hybrid journal bearings reveals that no comprehensive study has yet been carried out to investigate influence of wear, influence of non-Newtonian behavior of the lubricants, influence of surface roughness and thermal effects on the performance of multilobe non-recessed journal bearings. It is also observed that a detailed nonlinear transient response study for multilobe hole entry hybrid journal bearing system is needed to ascertain the dynamic response of a bearing system. Therefore, it is important to explore the individual/combined influence of wear, influence of non-Newtonian behavior of the lubricants, influence of surface roughness and thermal effects on the performance characteristics of journal bearings. Therefore, the work presented in this thesis is an attempt to bridge the gaps as indicated in literature review. The work reported in the present thesis has been carried out to accomplish the following objectives: i. To study the influence of wear on the performance of multilobe (two lobe and three lobe) non-recessed hybrid journal bearing system operating with Newtonian lubricant and compensated with various restrictors such as capillary, orifice, constant flow valve and slotentry. ii. To study the combined influence of wear and non-Newtonian lubricant on the performance of two lobe hybrid journal bearing system using different flow control devices such as capillary, orifice, constant flow valve and slot-entry journal bearing system. iii. To study the influence of surface roughness effect on the performance of multilobe (two lobe and three lobe) non-recessed hybrid journal bearing system using different flow control devices such as capillary, orifice, constant flow valve and slot-entry journal bearing system. viii iv. To study the combined influence of non-Newtonian behavior of the lubricant (cubic law lubricant) and thermal effect on the performance of two lobe hole-entry journal bearing system using different flow control devices such as capillary, orifice and constant flow valve. v. To predict the accurate response of journal center motions and decode the stability of two lobe hole entry hybrid journal bearing considering the influence of wear and surface roughness effect separately using non-linear model for motion trajectories for capillary restrictor and constant flow valve. The results in the present study have been numerically simulated by using the developed computer program and have been presented for the generally used values of bearing operating and geometric parameters such as bearing aspect ratio () = 1.0; land width ratio = 0.25; speed parameter () = 0.0,0.5,1.0; no. of rows of holes/slots = 2; no. of holes/slots per row = 6, 12; external load = 0.5-4.0; restrictor design parameter = 0.02-0.30; slot width ratio = 0.25; slot restrictor design parameter and The influence of wear is studied using the values of the wear depth parameter ( The effect of nonlinear behavior of the lubricant (cubic law lubricant) is studied using the values of non-linearity factor, To study the influence of surface roughness effect, the values of surface roughness patterns such as transverse , isotropic and longitudinal and the surface roughness parameter are used in the present work. A theoretical model is used to account and analyze the individual and/or combined influence of wear, non-Newtonian lubricant ( cubic law), surface roughness effect and thermal effect on the performance of bearing. The influence of wear is studied by considering different values of non dimensional wear depth parameter . A non-dimensional factor known as offset factor ( ) has been defined and used to account for the change in geometry of the circular bearing and nonlinearity factor is used to define the change in nonlinear behavior of the lubricant. The flow of an incompressible 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. Further, the Reynolds equations have been modified by considering the effect of surface roughness in the analysis of journal bearing system. A surface roughness model proposed by Patir and Cheng has been used in the present study. A thermohydrostatic solution for multilobe non-recessed journal bearing problem have been obtained by solving the relevant governing equations i.e. Reynold’s, Elasticity, Energy, Conduction equations.The modified Reynold’s equation is solved by taking the flow of ix lubricant through capillary, orifice, constant flow valve and slot restrictors as a constraint along with relevant boundary conditions. The system equation corresponding to the modified Reynold’s equation, after adjustment for the continuity of flow through the restrictors becomes non-linear for multilobe non-recessed journal bearing system compensated with an orifice restrictor operating with Newtonian and non-Newtonian lubricant. This non-linear equation, in the present work, has been 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 numerically simulated bearing performance characteristic parameters have been presented for capillary, orifice and constant flow valve compensated symmetric/asymmetric hole entry journal bearing configurations along with symmetric/asymmetric slot entry journal bearing configurations. The numerically simulated results presented in this study indicate that the performance characteristics of multilobe non-recessed journal bearing configurations are significantly affected by the wear defect, non-linear behavior of the lubricant, surface roughness orientation pattern and thermal effect. The present study further reveals a significant interaction between the influences of wear and non-Newtonian lubricant and thermal effect on the performance characteristics of a symmetric/asymmetric multilobe non-recessed journal bearing configurations. Thus, the influences of these effects should be studied individually and together for the realistic prediction of bearing performance characteristic data