Hydrodynamic Fluid Film Research Articles

Controllable Lubrication for Main Engine Bearings Using Mechanical and Piezoelectric Actuators

Although mechatronic systems are nowadays implemented in a large number of systems in vehicles, active lubrication systems are still incipient in industrial applications. This study is an attempt to extend the active lubrication concept to combustion engines and gives a theoretical contribution to this field. One refers to active lubrication when conventional hydrodynamic lubrication is combined with dynamically modified hydrostatic lubrication. In this study, two different schemes for the oil injection system in actively lubricated main engine bearings are presented. The use of active lubrication in journal bearings helps to enhance the hydrodynamic fluid film by increasing the fluid film thickness and consequently reducing viscous friction losses and vibrations. In this study, the hydrostatic lubrication is modified by injecting oil at controllable pressures through orifices circumferentially located around the bearing surface. The main equations that govern the dynamics of the injection for a piezo-actuated oil injector and a mechanical-actuated oil injector are presented. It is shown how the dynamics of the oil injection system is coupled to the dynamics of the bearing fluid film through equations. The global system is numerically solved using as a case study a single-cylinder combustion engine, where the conventional lubrication of the main bearing is modified by applying radial oil injection using piezo-actuated injection. The performance of such a hybrid bearing is compared to an equivalent conventional lubricated bearing in terms of the maximum fluid film pressures, minimum fluid film thicknesses, and reduction of viscous friction losses.

Rotordynamic Performance of an Oil-Free Turbo Blower Focusing on Load Capacity of Gas Foil Thrust Bearings

Engineered design of modern efficient turbomachinery based on accurate model predictions is of importance as operating speed and rate power increase. Industrial applications use hydrodynamic fluid film bearings as rotor support elements due to their advantages over rolling element bearings in operating speed, system stability (rotordynamic and thermal), and maintenance life. Recently, microturbomachinery (< 250 kW) implement gas foil bearings (GFBs) as its rotor supports due to its compact design without lubricant supply systems and enhanced stability characteristics. To meet the needs from manufacturers, the turbomachinery development procedure includes a rotordynamic design and a gas foil journal bearing (GFJB) analysis in general. The present research focuses on the role of gas foil thrust bearings (GFJBs) supporting axial load (static and dynamic) in an oil-free turbo blower with a 75 kW (100 HP) rate power at 30,000 rpm. The turbo blower provides a compressed air with a pressure ratio of 1.6 at a mass flow rate of 0.92 kg/s, using a centrifugal impeller installed at the rotor end. Two GFJBs with a diameter of 66mm and a length of 50 mm and one pair of GFTB with an outer diameter of 144 mm and an inner diameter of 74 mm support the rotor with an axial length of 493 mm and a weight of 12.7 kg. A finite element rotordynamic model prediction using predicted linearized GFJB force coefficients designs the rotor-GFB system with stability at the rotor speed of 30,000 rpm. Model predictions of the GFTB show axial load carrying performance. Experimental tests on the designed turbo blower; however, demonstrate unexpected large amplitudes of subsynchronous rotor lateral motions. Post-inspection reveals minor rubbing signs on the GFJB top foils and significant wear on the GFTB top foil. Therefore, GFTB is redesigned to have the larger outer diameter of 166 mm for the enhanced load capacity, i.e., 145%, increase in its loading area. The modification improves the rotor-GFB system performance with dominant synchronous motions up to the rate speed of 30,000 rpm. In addition, the paper studies the effect of GFTB tilting angles on the system performance. Insertion of shims between the GFTB brackets changes the bearing tilting angles. Model predictions show the decrease in the thrust load capacity by as large as 86% by increase in the tilting angle to 0.0006 rad (0.03438 deg). Experimental test data verify the computational model predictions.

Investigation of the Influence of Volute Design on Journal Bearing Bias Force Using Computational Fluid Dynamics

Hydrodynamic fluid film bearings represent an optimal possibility for rotary blood pump (RBP) miniaturization and wear-free operation. Size is a key parameter in the development of ventricular assist devices (VADs) as smaller patients and the pediatric population become eligible for the device. In order to maintain rotor suspension, radial journal bearings have been widely used in industrial applications as well as in some VADs. A main influence on the performance of such a bearing is the applied hydraulic bias force. This study combines numerical and analytical approaches to determine the bias force of different impeller-volute configurations and the resulting eccentricity for the hydraulic design point and also for off-design operation. Significant differences occur for different impeller-volute configurations, with the circular volute displaying the most beneficial properties for a stable impeller suspension. Moreover, an analytical prediction of eccentricity was found to be incorrect for the relatively small forces that occur in RBPs.

Vibration Characteristics of Hydrodynamic Fluid Film Pocket Journal Bearings

Theoretical analyses of hydrodynamic fluid film bearings with different bearing profiles rely on solutions of the Reynolds equation. This paper presents an approach used for analysing the so‐called pocket bearings formed from a combination of offset circular bearing profiles. The results show that the variation of the dynamic bearing characteristics with different load inclinations for the pocket bearings is less than that for the elliptic bearing counterpart. It is shown that the natural frequencies as well as the critical speeds, and hence the vibrational behaviour, can also be significantly different for an industrial rotor supported by the different bearings.

Lubrication of Silicon Nitride and Silicon Carbide by Water: Running in, Wear and Operation of Sliding Bearings

The purpose of this work was to establish the conditions for the operation and break-in of water-lubricated ceramic bearings. The experiments consisted of sliding 1/4″ silicon nitride or—carbide balls against pre-polished disks of the same material in water until tribochemical wear generates smooth conformal surfaces that allow hydrodynamic lubrication (μ<0.002) by very thin water films. This “running in” was performed at various sliding speeds (0.01-4m/s) and loads (0.5-20N). The minimum sliding speed for low friction were 0.04m/s for silicon nitride and 0.5m/s for silicon carbide, much lower than for conventional bearings. The load carrying pressures were 60-80MPa, which is higher than the usually pressures of thrust bearings. The hydrodynamic fluid film thickness was estimated with a standard integration of Reynolds' equations modified for circular geometry, it was to be 5-15nm for silicon nitride, 25nm for silicon carbide. Operation over long distances (80km) allowed us to measure the wear rate during hydrodynamic lubrication; this was found to be <2×10−11mm3/nm, a rate acceptable for industrial application. A novel method completed during this work allows the determination of the wear rate during run-in. It varies with sliding velocity for silicon nitride, from 1 to 6×10−5mm3/nm; it is constant at 4×10−6mm3/nm for silicon carbide.

Theoretical Studies of a Continuously Adjustable Hydrodynamic Fluid Film Bearing

Principles of a continuously adjustable hydrodynamic bearing are described together with an analysis model for studying its theoretical performance. The model included an expanded form of the governing Reynolds equation which took account of non-uniform variations in the fluid film thickness. A solution procedure was devised whereby for a given set of adjustment conditions, simultaneously converged fields of fluid film thickness, temperature, viscosity and pressure would result, together with oil film forces. A wide range of operating characteristics were studied with results predicting advantages and benefits over conventional hydrodynamic bearings.

Thermal Analysis of Flow in the Deformation Phase of Liquid Lubricated Plane Strain Forging

The generalized energy equation is reduced into a simple form for conduction and convection modes of heat transfer across a hydrodynamic incompressible fluid film in the deformation phase of plane-strain forging. Expressions for the temperature, velocity, and flow distributions are obtained by solving the energy equation with appropriate boundary conditions. Application of this analysis is illustrated for the film formation process in the deformation phase of liquid lubricated plane strain forging. The analysis indicates that the Peclet number plays an important role in deciding the variation of lubricant film thickness with position and time in the deformation phase. [S0742-4787(00)02504-2]

A mathematical model and numerical solution technique for a novel adjustable hydrodynamic bearing

An analysis model for a novel adjustable hydrodynamic fluid film bearing is described. The principles of hydrodynamic lubrication are outlined together with an expanded version of the governing pressure field equation as related to the novel bearing. Finite difference approximations are given for the pressure field equation and a temperature model, both related to the fluid film thickness. Relationships of viscosity with temperature and pressure are included. A finite element model and an iterative computational process are described, whereby full simultaneously converged field solutions for fluid film thickness, temperature, viscosity and pressure were obtained, together with oil film forces. The model and solution process were developed to apply to a variety of hydrodynamic bearings and an outline is given of its extensive use in the design and simulation of one version of the novel bearing. Observations are given on the operation, success rates and verifications of the computational process. Copyright © 1999 John Wiley & Sons, Ltd.

A mathematical model and numerical solution technique for a novel adjustable hydrodynamic bearing

An analysis model for a novel adjustable hydrodynamic fluid film bearing is described. The principles of hydrodynamic lubrication are outlined together with an expanded version of the governing pressure field equation as related to the novel bearing. Finite difference approximations are given for the pressure field equation and a temperature model, both related to the fluid film thickness. Relationships of viscosity with temperature and pressure are included. A finite element model and an iterative computational process are described, whereby full simultaneously converged field solutions for fluid film thickness, temperature, viscosity and pressure were obtained, together with oil film forces. The model and solution process were developed to apply to a variety of hydrodynamic bearings and an outline is given of its extensive use in the design and simulation of one version of the novel bearing. Observations are given on the operation, success rates and verifications of the computational process. Copyright © 1999 John Wiley & Sons, Ltd.

Non-linear dynamics of a rigid, unloaded andunbalanced rotor on lubricated bearings

The dynamic behaviour and stability analysis of an unloaded rotoron cylindrical lubricated bearings subjected only to the actionof unbalance is presented. The ``short bearing model'' has beenadopted to evaluate the hydrodynamic fluid film force. As theunbalance increases, the system shows the typical behaviour ofnon-linear mechanical systems as the non-linearity of the hydrodynamic force field becomes more important. Evaluating theapproximate circular and centred analytical expressions of thetrajectory, the frequency response curves of the amplitude ofthe synchronous orbit have been reported. The stability limitcurve of the synchronous response has been obtained by means ofthe Floquet theory. Several examples of journal trajectories,obtained through step-by-step integration of the motionequations, are reported to illustrate the dynamic behaviour ofthe system and to verify the validity of the stability limitcurve.

A model of the dynamics of boundary film formation

The dynamics of formation and loss of the boundary films formed during sliding on steel surfaces were investigated over a range of temperature. Tests are performed on a cylinder-on-disk machine using mineral oil with various concentrations of zinc dialkyldithiophosphate (ZDP). The thickness and refractive index of the boundary films during step load test were monitored in situ with an ellipsometer, and the composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS). As temperature increases, chemical reactivity increases the film formation rate, while the film removal rate increases owing to (a) the decrease of durability of the boundary film material and (b) the reduction of hydrodynamic fluid film thickness due to decreasing viscosity of the lubricant. There is a balance between these two competing mechanisms, and this balance is reflected in the boundary film thickness. The boundary films consist of a film of oxide and metallic compound (OMM) covered by an organo-iron compound (OIC). Their relative effectiveness in preventing scuffing depends on temperature and composition. In particular, the OIC is effective in reducing wear of the opposing surfaces by covering the OMM.

The Rheology and Hydrodynamics of Dry Powder Lubrication

This paper conceptualizes a powder lubrication mechanisms which closely resembles that of a hydrodynamic fluid film. Based on the observations of past investigations and on the author's experiments, it is postulated that a layered shearing of the compacted powder generates velocity, density, and temperature profiles akin to fluid film bearings. Thus, a lubricant consisting of a fine powder unserted either deliberately or one generated by the water of the mating surfaces, constitutes a viable lubricant that generates the required flows and pressures to prevent contact between the surfaces.

Elastic Bearings Lubricated With Non-Newtonian Power Law Fluids—A Boundary Element Approach

This paper discusses the influence of two material property effects on the hydrodynamic lubrication of large-slenderness-ratio, plane slider bearings. Bearing surfaces are covered with an elastic material. This uniformly thick elastic material deforms under the hydrodynamic fluid film pressure. The elastic layer is incompressible (i.e. Poisson ratio is ½). The lubricant is a power-law, non-Newtonian fluid with shear thinning properties. A boundary element method is wed to determine the elastic solid deformation. The important dimensionless parameter coupling the elastic deformation and hydrodynamic pressure is the deformation number. A comparison of bearing pressure profiles and bearing gap-geometries for different fluids is shown. Results reveal increased bearing gaps due to deformation, gap geometries that are more uniform in thickness due to fluid film pressure, pressure peaks moving rearward with increased bearing deformation, sharper converging gaps formed near the bearing trailing edge, and effective hydrodynamic-wedge angles that are decreased by bearing surface deformation.

Viscous incompressible flow in a narrow channel with one free wall and fluid supply through a porous insert

The problem investigated relates the plane unsteady flow of a viscous incompressible fluid in a narrow channel one of whose walls is free and acted upon by a given load, while the other is rigidly fixed. The fluid enters the channel through a porous insert in the stationary wall. A model of the flow of a thin film of viscous incompressible fluid and Darcy's law for flow in a porous medium are used to find the distribution of fluid pressure and velocity in the channel and the porous insert in the two-dimensional formulation for fairly general boundary conditions in the case where the length of the porous insert exceeds the length of the free wall. In the particular case where the length of the porous insert is equal to the length of the free wall an exact stationary solution of the problem is obtained for a given value of the channel height. The stability of the equilibrium position of the free wall supported on a hydrodynamic fluid film is examined.

Vibratory actuator incorporating hydrodynamic journal bearing

A vibratory actuator (22) includes a housing (62), a rotatable shaft, (78) extending through the housing, and an eccentric member (80) to which the shaft is secured. Vibration is effected upon rotation of the eccentric member (80) by the shaft (78). A hydrodynamic fluid film gap is provided between a partial cylindrical journal surface of the eccentric member (80) and a full cylindrical bearing surface of the housing (62), and the housing contains hydrodynamic fluid. Radial loading between the housing (62), shaft (78) and eccentric member (80) is more uniformly distributed to reduce structural deflections and thus facilitate maintenance of the proper film of hydrodynamic fluid during operation of the actuator (22). If desired, the fluid can be circulated through the housing (62) for cooling purposes.

Zero-load stability of a rigid rotor supported on pressurized porous gas bearings

A theoretical and experimental investigation of stability characteristics of a rigid rotor mounted on externally expressurized gas-lubricated porous bearings in made. The hydrodynamic fluid film forces of an unloaded journal which undergoes steady whirl are obtained to find limits of the stable region. The results are compared with experimental data and with a similar solution using Galerkin's method. The effect of a feeding parameter, supply pressure, thickness of porous bushing and porosity of bushing material on stability characteristics is also investigated.

Preliminary Investigation of Minimum Oil-Feed Rates for Fluid-Film Conditions in Journal Bearings

Abstract Many industrial bearings do not operate with an abundant supply of lubricant. Bearings that are fed with wicks, pads, and drop-feed oilers, for example, are found in this category. It has been demonstrated that a certain volume of lubricant must be supplied to a journal bearing to permit the formation of a complete hydrodynamic fluid film. Since it is generally desirable to have bearings operate with a fluid film in order to develop the least friction and wear, the question of determining the minimum rate of feed in order to insure such operation is of some importance. The work described in this paper has resulted in a method which appears promising for predicting the minimum feed rate for a journal bearing. The minimum feed rate is evaluated in terms of speed, load, clearance, and size of bearing. The method is based on a theoretical analysis supplemented by experimentally determined constants.