A STUDY OF NON-RECESSED HYBRID JOURNAL BEARINGS WITH TEXTURED SURFACES

Abstract

Bearings are essential elements of a machine and its performance affects the overall health of machine. Without the proper functioning of bearing elements, proper and smooth operation of machinery is not feasible. Thus, their careful selection, application and operation is a crucial step in the rotor-bearing systems. For heavier load applications such as power plant, crusher, aerospace, turbo machinery, process industries etc., fluid film bearings are extremely useful all over the world. In order to get an optimum performance of machinery, the fluid film bearings must be designed more accurately and on the basis of more realistic operating conditions. Last few decades have witnessed more focused in the area of fluid film hydrostatic/hybrid journal bearing system so as to cope with stringent requirement of modern day machinery. The use of journal bearing configurations increased greatly in high speed heavily loaded machines. This led the bearing designer to develop other bearing configurations to operate under stringent operating requirements with improved performance. Therefore, the hybrid journal bearings have been developed and are being used successfully in machines, which operates under high speed and heavily loaded conditions. The hybrid journal bearings are widely used 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 equipment etc. The hybrid fluid film journal bearings are generally classified in two categories as recessed/pocket hybrid journal bearing and non-recessed hybrid journal bearing. In the conventional recessed hybrid journal bearings, the recesses/pockets occupy a considerable land area of total bearing area and thus, leaving less area for hydrodynamic action when operating under speed (hybrid mode). Therefore, to overcome this limitation the bearing designers make use of non-recessed hybrid journal bearing configurations and used efficiently to enhance bearing performance in respect to recessed bearings under high speed and heavy load conditions. Further, circular hybrid journal bearings exhibit the problem of fluid induced instability in whirl or whip, when operating at high speeds. When the amplitude of whirl oscillation grows too large and the bearing becomes unstable. It may cause serious vi damage such as loss of babbitt on the bearing surface, seal failure etc. Thus, the bearing designers/researchers developed the idea of non-circular (multi-lobe) profile journal bearing and successfully employed this journal bearing to handle the problem of fluid induced instability at high speeds. The non-circular (multi-lobe) bearings are extensively used in high speed application in industry, as they are simple, efficient and economically viable and provide improved stability. In an actual practice, the tribo-contacts usually have irregularities owing to inherent limitation of machining process, abrasive wear, impulsive damages, rust etc. Due to the random nature of the surface, the local fluid film thickness contributes to the fluid film pressure as a random quantity for roughened tribo-contact (bearing) systems. Realizing this fact, the bearing designers were tempted to fabricate regular patterns of micro-dimples (surface texturing) on the surface of bearing and they obtained remarkable improvements in the performance of bearings. The textured surfaces offers great versatility on the improvement of tribological performance of fluid film journal bearings i.e. dimple acting as micro-reservoir to storing and providing the fluid directly to the contact zone, increasing the fluid film thickness between tribo-contacts consequences reducing the frictional coefficient, providing an additional micro-hydrodynamic lift effect to increasing the load carrying capacity. Micro-dimples also trap wear particles to minimize further abrasion and increase service life of the bearing. The lubricant behaviour plays a very vital role on the performance of fluid film journal bearings systems. Nowadays, the tribologists/lubricating engineers quite often blends the different type of external organic or inorganic additives in base oil to improve the lubricating performance of lubricants. The additization of these types of additives cause the base oil lubricant to behave as a non-Newtonian lubricant. The classical theory of Newtonian fluids neglect the size of fluid particles, it fails to describe the behavior of non-Newtonian fluids such as polymer thickened oils, lubricants with various additives, synthetic and bio-fluids. Generally, these long-chain organic compounds (additives) has the value of length could be a million times the diameter of a water molecule, thus, it affects the performance of the bearing significantly. Thus, the size of long-chain polymer molecules (additives) has been considered in couple stress fluid model. Similarly, the micro-rotation and micro-translation of suspended fluid particles are considered in micropolar fluid model. Nowadays, for a new generation of machines, the electro-rheology lubrication technology has emerged as an effective smart way to enhance the performance of tribo-contacts (fluid film bearings) with adjustable dynamic characteristics. Thus, in the present work, four different types of non-Newtonian lubricants such power law, couple stress, micropolar and electro-rheological lubricants vii have been used to investigate the performance analysis of textured/non-textured circular/non-circular non-recessed hybrid journal bearing systems. The performance of hydrostatic/hybrid journal bearing is strongly dependent on types of flow control device/restrictor used. It adjusts the variations in supply pressure corresponding to variation in external load and compensates the appropriate lubricant flow rate. The lubricant is supplied at some other pressure, necessitating a pressure control device or restrictor between the supply and pocket to provide fluid film stiffness. The suitable values of fluid film dynamic coefficient (stiffness/damping) are often necessary in machine tools and other precision machinery from the view point of dynamic performance. Therefore, the correct selection of suitable compensating device enables high bearing stiffness, high load capacity and protection against loss of load capacity due to variations within the manufacturing tolerances. An extensive literature survey of fluid film journal bearing systems done in this work indicates that the studies dealing with non-circular bearing and textured bearing configuration are mainly confined to hydrodynamic journal bearing systems. Some limited research works concerning the non-circular (multi-lobe) non-recessed hydrostatic/hybrid journal bearing configurations have been reported in literature. To the best knowledge of author, no study is yet available concerning the textured hydrostatic/hybrid non-recessed journal bearing systems. Therefore, the present work is aimed to investigate the individual/combined influence of textured surfaces, influence of non-Newtonian behavior of the lubricants, influence of non-circular profile of bearings and influence of flow compensating methods on the performance characteristics of non-recessed hybrid journal bearings. The work presented in this thesis is an attempt in above direction to achieve the following objectives: (i) To study the influence of various type of textured surface patterns (full textured, partial textured first portion, partial textured second portion) and shapes (circular, conical, triangle, spherical and ellipse) on the performance of non-recessed hybrid journal bearing systems. (ii) To investigate the individual/combined influence of textured surface, non-circular profile of bearing (two-lobe and three-lobe) and non-Newtonian lubricants (power law, couple stress, micropolar and electro-rheological lubricant) on the performance of non-recessed (hole-entry/slot-entry) hybrid journal bearing systems (iii) To investigate the influence of various types of compensating elements (restrictors) such as capillary, orifice, CFV and slot-entry, on the performance of textured/non-textured circular/non-circular (two-lobe, three-lobe) non-recessed hybrid journal bearing systems. viii To analyze the performance of textured/non-textured circular/non-circular non-recessed hybrid journal bearings, the modified Reynolds equation governs the flow of an incompressible Newtonian/non-Newtonian (power law, couple stress, micropolar, ER) lubricant in the bearing clearance space of fluid film hybrid journal bearing. In the present work, the finite element method (FEM) has been used for deriving global system of equations. Further, to solve set of equations used in this study, a computer program has been developed in MATLAB using a solution algorithm and iterative schemes. Once the equilibrium position of journal center  ,  JJXZ is established, the nodal fluid film pressure distribution is simulated. Based on the computed values of nodal fluid film pressures, the performance characteristics of textured non-recessed hybrid journal bearing are numerically simulated. In the present study, MATLAB computer program has been developed and is used to numerically simulate the bearing performance characteristics for the generally used values of bearing operating and geometric parameters such as bearing aspect ratio () = 1.0; land width ratio (𝑎̅𝑏 ) = 0.25; speed parameter () =1.0; no. of rows of holes/slots = 2; no. of holes/slots per row =12; external load (𝑊 ̅ 𝑜) = 1.0-1.6; restrictor design parameter (𝐶̅ 𝑠2) = 0.0873, 0.0614, 0.0438; slot width ratio (SWR) = 0.25; and offset factor of non-circular bearing (𝛿) = 0.9, 1.0, 1.1. Further, the influence of textured surfaces is studied using the values of the dimple depth parameter (ℎ̅ 𝑝) = 0.0 − 2.0). The influence of nonlinear behavior of the power-law lubricant is studied using the values of power law index, 𝑛 = 0.7, 1.0, 1.3. The effect of micropolar lubrication is studied using the values of coupling number, 𝑁2 = 0.0, 0.3, 0.7 and characteristics length of micropolar lubricant, 𝑙̅𝑚 = ∞, 10, 20. The effect of couple stress lubrication is studied using the values of couple stress parameter, 𝑙̅𝑐 = 0.0, 0.2, 0.4.The effects of ER fluid lubrication have been studied by considering the different values of external applied voltages on the ER fluid, 𝑉 = 0.0, 600, 1200 𝑉𝑜𝑙𝑡𝑠, and constant ER fluid parameter, B = 2.185. These operating and geometric parameters have been taken from already published literature. The theoretical models have been developed to account and analyze the individual and/or combined effects of textured surfaces, non-Newtonian lubricants (power-law, couple stress, micropolar and electro-rheological), non-circular profile of bearing and various methods of compensation, on the performance of non-recessed hybrid journal bearing. In this thesis, the bearing static performance characteristics parameters in terms of fluid film pressure distributions( p ), nominal minimum fluid film thickness( min h ), frictional torque( f T ) and lubricant flow (Q ), and dynamic performance ix characteristics parameters which includes fluid film stiffness, ) , , ( z x j i Sij  / damping ) , , ( z x j i Cij  coefficients, stability threshold speed margin ( th ) as well as linear/non-linear trajectories, have been numerically simulated for the generally used representative values of bearing operating and geometric parameters. The numerically simulated results in this study reveals that the bearing performance characteristics parameters of non-recessed hybrid journal bearing are enhanced by considering spherical textured surface on bearing surfaces together with the use of various types of non-Newtonian lubricants (couple stress, micropolar and electro-rheological), use of non-circular (two-lobe/three-lobe) profile of bearing and bearing compensated with CFV flow control devices. The results presented in this thesis are expected to be quite useful for the bearing designers as well as to the academic community to design an efficient fluid film journal bearing system.