Increase In Minimum Film Thickness Research Articles

Behavior of elastohydrodynamic lubricating film point contact problems in the unsteady state of reciprocating motion

Purpose In this study a numerical analysis of the elastohydrodynamic lubrication point contact problem in the unsteady state of reciprocating motion is presented. The effects of frequency, stroke length and load on film thickness and pressure variation during one operating cycle are discussed. The general tribological behavior of elastohydrodynamic lubrication during reciprocating motion is explained. Design/methodology/approach The system of equations of Reynolds, film thickness considering surface deformation and load balance equations are solved using the Newton-Raphson technique with the Gauss-Seidel iteration method. Numerical solutions were performed with a sinusoidal contact surface velocity to simulate reciprocating elastohydrodynamics. The methodology is validated using historical experimental measurements/observations and numerical predictions from other researchers. Findings The numerical results showed that the change in oil film during a stroke is controlled by both wedge and squeeze effects. When the surface velocity is zero at the stroke end, the squeeze effect is most noticeable. As the frequency increases, the general trend of central and minimum film thickness increases. With the same entraining speed but different stroke lengths, the properties of the oil film differ from one another, with an increase in stroke length leading to a reduction in film thickness. Finally, the numerical results showed that the overall film thickness decreases with increasing load. Originality/value General tribological behaviors of elastohydrodynamic lubricating point contact, represented by pressure and film thickness variations over time and profiles, are analyzed under reciprocating motion during one working cycle to show the effects of frequency, stroke length and applied load.

Investigation on the effect of coating properties on transient mixed lubrication of gas foil bearing

To reveal the effect of coating properties on the mixed lubrication evolution during the start–stop process of gas foil bearing (GFB), a transient gas mixed lubrication-wear model for coating surface is established. In the developed model, a method of the extended equivalent modulus by the film thickness ratio is proposed, which can extend the classic Greenwood and Tripp contact model for homogeneous materials to the coating contact involving layered materials. Moreover, a gas foil bearing start–stop experiment is conducted to verify the predicted results of the developed model. The effects of coating materials and standard deviation of coating surface on the start–stop performance of GFB are investigated. Within the scope of this article, the results show that during start-up, the gas film pressure first increases and then decreases with the increase in time, the minimum film thickness increases, while the asperity contact decreases. The opposite evolution is observed during shut-down. Compared with uncoated GFB, the friction torque and lift-off speed of GFB with polyimide (PI) and polytetrafluoroethylene (PTFE) are reduced, with PI reducing friction torque by 24.27% and PTFE by 49.03%. The minimum lift-off speed with PTFE is the smallest. The lift-off speed, friction torque, and wear volume increase with the increase in the standard deviation of coating surface. The friction torque and lift-off speed decrease as the start–stop cycle increases until both reach stability. This paper can provide theoretical guidance for the design of the top foil coating surface of GFB.

Study on transient performance of tilting-pad thrust bearings in nuclear pump considering fluid-structure interaction

To study the lubrication performance of tilting-pad thrust bearing (TPTBs) during start-up in nuclear pump, a hydrodynamic lubrication model of TPTBs was established based on the computational fluid dynamics (CFD) method and the fluid-structure interaction (FSI) technique. Further, a mesh motion algorithm for the transient calculation of thrust bearings was developed based on the user defined function (UDF).The result demonstrated that minimum film thickness increases first and then decreases with the rotational speed under start-up condition. The influence of pad tilt on minimum film thickness is greater than that of collar movement at low speed, and the establishment of dynamic pressure mainly depends on pad tilt and minimum film thickness increases. As the increase of rotational speed, the influence of pad tilt was abated, where the influence of the moving of the collar dominated gradually, and minimum film thickness decreases. For TPTBs, the circumferential angle of the pad is always greater than the radial angle. When the rotational speed is constant, the change rate of radial angle is greater than that of circumferential angle with the increase of loading forces. This study can provide reference for improving bearing wear resistance.

Combined effect of circumferential roughness and squeeze velocity on ferrofluid squeeze film with rotating sphere and rough plate

Based on the Jenkins model for ferrofluid flow regarding the performance of squeeze-film lubrication between rotating upper spherical surface and circumferentially rough lower plate is presented. Here, for hydrodynamic lubrication, we have adopted the ferrohydrodynamic theory. Further, using Christensen’s stochastic model, surface roughness is examined. In the paper, in the presence of a radially variable magnetic field, rotations of both the upper and lower surfaces as well as roughness of the surface, the Reynolds type equation for the squeeze-film bearing is established. By using the Reynolds type equation, expressions for the non-dimensional film pressure and load-carrying capacity are obtained for circumferential roughness pattern. Neuringer–Rosensweig (NR) model results are computed using the Jenkins model. The results show that non-dimensional film pressure increases remarkably when the non-dimensional radial parameter ( R) increases. Also, as the nominal minimum film thickness increases, load-carrying capacity decreases in the Jenkins model. However, as a result of Tables 5 and 6, the non-dimensional load-carrying capacity of the Jenkins model is improved compared to that of the NR model.

Transitional stages of thin air film entrapment in drop-pool impact events

Abstract

Performance analysis of thrust pad bearing using micro-rectangular pocket and bionic texture

This paper presents the performance behaviors of a hydrodynamic thrust bearing incorporating new pad surface designs involving microrectangular pocket and Labeo rohita fish scale texture. The bearing performance parameters have been numerically explored and analyzed using three different pad surface designs. The effects of texture parameters such as pocket/texture depth, extend of pocket, and fish texture spread in the circumferential and radial directions, the number of scale texture waves in circumferential and radial directions, have been investigated for improving the performance of the bearing. It has been found that the pad surface having microrectangular pocket and fish scale texture together yields substantial increase in minimum film thickness and reduction in friction coefficient as compared to the other pad designs.

Performance behaviours of a self-acting gas journal bearing with a new bore design

A new bore design of a self-acting gas journal bearing has been conceived for enhancing the load carrying capacity and dynamic stability parameters as compared to conventional bump-foil bearing. The new bore design involves the synergistic presence of bump foil and rigid sector segments. The governing equations have been discretized employing FVM/FDM followed by the solutions of the algebraic equations using Gauss-Siedel iterative scheme. Based on the investigations performed with new bore, 5–260% increase in minimum film thickness and 80–175% improvement in stability parameters have been achieved for the range of operating parameters adopted in present study in comparison to conventional bump-foil journal bearing.

Journal Bearing Surface Topography Design Based on Transient Lubrication Analysis

Abstract A transient mixed lubrication model is used to study the performance of a journal bearing subjected to impulse loading, considering mass conservation and the effects of asperities on flow and contact, to explore a novel journal bearing surface design methodology. The bearing surface features include an outlet pocket and axial lip for topographical design consideration. A data-driven approach for a steadily loaded bearing was first presented to illustrate the behavior of an indented pocket design at low and high loads, and Pareto optimization and sensitivity analysis methods were employed to analyze the data and provide insight to the design. The results show that the pocket location was the most influential parameter for the optimal bearing surface design for energy-efficient lubrication performance. For transient operation, a comprehensive parametric study was conducted, and the results reveal that, when compared to the results of the unmodified bearing, the bearing designed with the optimal outlet pocket can run at 9% lower mean friction while maintaining the baseline film thickness. The bearing with the lip feature shows a 20% minimum film thickness increase and 3% mean friction reduction. The design with outlet pocket and lip features combined can lead to 20% film thickness increase and 10% mean friction reduction.

A novel numerical scheme for the analysis of effects of surface roughness on EHL line contact with couple stress fluid as lubricant

In this article, a Jacobian-free Newton Multigrid (JFNMG) method is used for obtaining the solution of isothermal, steady and compressible elastohydrodynamic lubrication (EHL) line contact problem with surface roughness. The lubricant is a couple stress fluid. A finite difference scheme is used for the solution of EHL equations. The proposed JFNMG method, for the solution of resulting nonlinear system of algebraic equations, comprises nonlinear Newton iterations on the outer loop and linear multigrid iterations on the inner loop. It overcomes the limitations of conventional schemes for the investigation of the problems covering wide range of physical parameters of interest. For increasing values of couple stress parameter, there is an increase in minimum film thickness and considerable decrease (in height as well as spread) of pressure spike. Also, the sensitivity of height and spread of pressure spike as a function of load, couple stress parameter and other parameters are investigated.

Rough Soft-EHL with Non-Newtonian Lubricant

This paper presents the effect of surface roughness on soft elastohydrodynamic lubrication in circular contact with non-Newtonian lubricant. The time independent modified Reynolds equation, elastic equation and lubricant viscosity equation were formulated for compressible fluid. Perturbation method, Newton-Raphson method, finite different method and full adaptive multigrid method were implemented to obtain the film pressure, film thickness profiles and friction coefficient in the contact region at various the amplitude of surface roughness, surface speed of sphere, modulus of elasticity and radius of sphere. The simulation results showed that the film thickness in contact region depended on the profile of surface roughness. The minimum film thickness decreased but maximum film pressure and friction coefficient increase when the amplitude of surface roughness and modulus of elasticity increased. For increasing surface speeds, the minimum film thickness and friction coefficient increase but maximum film pressure decreases. When radius of sphere increases, the minimum film thickness increases but maximum film pressure and friction coefficient decrease.

Effects of Surface Roughness on Elastohydrodynamic Lubrication in Elliptical Contact with Non-Newtonian Fluids

This paper presents the effect of surface roughness on the performance characteristics of elastohydrodynamic lubrication with non-Newtonian fluid base on Carreau viscosity model in elliptical contact. The time independent modified Reynolds equation and elastic equation were formulated for compressible fluid. Perturbation method, Newton Raphson method and full adaptive multigrid method were implemented to obtain the film pressure, film thickness profiles and friction coefficient in the contact region at various amplitude of combined surface roughness, applied loads, speeds and elliptic ratio. Simulation results show surface roughness amplitude has significant affected the film pressure in the contact region. The minimum film thickness decreases but friction coefficient increases when the combined roughness and applied loads increases. The minimum film thickness and friction coefficient both increase as the relative velocity of the ball and the plate is increase. For increasing the elliptic ratio, the minimum film thickness increases but the friction coefficient decreases.

The Influence of Surface Tension on Lubrication Film Thickness and Pressure

Modified Reynolds equation is deduced considering surface tension in this paper. The influence of surface tension on lubrication and elastohydrodynamic lubrication is analyzed. Result shows surface tension has apparent influence on oil film thickness. It makes minimum film thickness increase under relative thin film thickness. The influence decreases rapidly with the increasing of film thickness. Surface tension has little influence on oil film pressure distribution.

Research on the Effect of Whole Cylinder Block on EHL Performance of Main Bearings Considering Crankshaft Deformation for Internal Combustion Engine

In this paper, the analyses of elastohydrodynamic lubrication (EHL) of crankshaft bearings considering the deformation of the whole cylinder block and crankshaft under load were carried out for the crankshaft bearing system of a four-stroke four-cylinder internal combustion engine. The lubrication of crankshaft bearing was analyzed by dynamic method. The deformation of bearing surface under pressure of oil film was calculated by compliance matrix method. The results show that when the crankshaft deformation under load is considered, compared with the results of not considering the deformation of cylinder block, the maximum film pressure decreases, the minimum film thickness increases, and the end leakage flow-rate and frictional coefficient of journal surface change little in an engine working cycle when considering the deformation of cylinder block. The models of the whole cylinder block and the single main bearing housing were used, respectively, to calculate the deformation of main bearing surface in the analyses. The results show that the calculation accuracy of the elastohydrodynamic lubrication analyses of crankshaft main bearings can be met basically by applying the simple model based on the single main bearing housing to calculate the elastic deformation of main bearing surface.

Effect of Stiff Coatings on EHL Film Thickness in Point Contacts

Coatings are widely used for interface performance enhancement and component life improvement, as well as for corrosion prevention and surface decoration. More and more mechanical components, especially those working under severe conditions, are coated with stiff (hard) thin coatings. However, the effects of coatings on lubrication characteristics, such as film thickness and friction, have not been well understood, and designing coating for optimal tribological performance is a grand challenge. In this paper, the influences of coating material properties and coating thickness on lubricant film thickness are investigated based on a point-contact isothermal elastohydrodynamic lubrication (EHL) model developed recently by the authors. The results present the trend of minimum film thickness variation as a function of coating thickness and elastic modulus under a wide range of working conditions. Curve fitting of numerical results indicates that the maximum increase in minimum film thickness, Imax, and the corresponding optimal dimensionless coating thickness, H2max, can be expressed in the following forms: Imax=0.769M0.0238R20.0297L0.1376exp(−0.0243ln2L) and H2max=0.049M0.4557R2−0.1722L0.7611exp(−0.0504ln2M−0.0921ln2L). These formulas can be used to estimate the effect of coatings on film thickness for EHL applications.

Surface roughness effects in elastohydrodynamic lubrication line contacts using couple stress fluid

The effect of couple stress fluids on elastohydrodynamic lubrication (EHL) behaviour in smooth as well as rough line contacts is investigated numerically at low speed-high load and high speed-low load conditions. A non-dimensional couple stress parameter represents the molecular length of the additives. The EHL characteristics computed for couple stress fluids are found to have strong dependence on couple stress parameter. Significant increase in minimum film thickness due to couple stress fluid is observed. Also, the surface roughness effects on EHL behaviour are influenced by couple stress fluid to an extent depending upon the speed-load conditions.

Effects of couple stresses and surface roughness on the film thickness of heavily loaded rollers

PurposeThis paper aims to study the effects of couple stresses and surface roughness on the minimum film thickness of heavily loaded rollers and to discuss these by following Grubin's approach and Crook's approximations.Design/methodology/approachA generalised form of Reynolds equation for rough surfaces with lubricant as couple stress fluid is derived. This equation is then used to study the combined effect of couple stresses and surface roughness on the roller bearings under heavily loaded conditions. EHD minimum film thickness expressions is obtained by following Grubin's approach and Crook's approximation and it is studied numerically.FindingsIt is found that, as the chain length of the additive molecules increases, the elastohydrodynamic minimum film thickness increases. Also, as the mean height of roughness asperities increases, the elastohydrodynamic minimum film thickness increases for the transversal roughness and it decreases in the case of longitudinal roughness.Research limitations/implicationsThese effects are studied theoretically by the mathematical equations in heavily loaded roller bearings.Practical implicationsReduction of the film thickness in the EHD lubrication between the rollers can be compensated by the use of lubricants containing additives of molecules of size. As a result the bearing performance can be improved.Originality/valueThis research paper provides a closed form of the expressions for the bearings in EHD lubrication and is studied with regard to couple stress parameter. This paper helps to manufacture better bearings.

Transient elastohydrodynamic lubrication analysis of an involute spur gear using couple-stress fluid

A complete numerical solution to the transient elastohydrodynamic lubrication (EHL) problem of an involute spur gear is obtained under isothermal condition. The lubricant is assumed to be couple-stress fluid and the transient Reynolds equation for the compressible couple-stress fluid is derived using Stokes theory. The EHL characteristics computed for couple-stress fluids are found to have strong dependence on couple-stress parameter. Significant increase in minimum film thickness due to couple-stress fluid is observed. Also, it is found that couple-stress fluids tend to smoothen the variation of film thickness along the line of action caused by transient effects.

Journal bearings optimization for mass balancing systems used in internal combustion engines

PurposeTo provide an optimization methodology for the journal bearings of mass balancing systems, that consists in a sensibility analysis of the journal bearings design parameters in the journal bearing operating parameters, as well as the comparison and selection of the journal bearing materials and the selection of the lubrication grooves suitable.Design/methodology/approachThe methodology followed comprehends three steps. The first step is the one by one variation of the journal bearing design parameters (radial clearance, journal bearing diameter and journal bearing length) to analyse, independently, their influence on the journal bearing operating parameters (minimum film thickness, maximum pressure and power loss). The second step is the analysis and comparison of the metallic materials that can be used in the journal bearings. The third step is the selection of the lubrication groove.FindingsApplying this methodology it is shown that increasing the radial clearance the minimum film thickness increases and the maximum pressure and the power loss decrease, increasing the journal bearing length the minimum film thickness and the power loss increase and the maximum pressure decreases, at last, increasing the journal bearing diameter the minimum film thickness, the maximum pressure and the power loss increase. The materials that should be used are the white metals (Babbitts). And the journal bearing lubrication grooves should be circumferential.Practical implicationsThis method gives the user the possibility to eliminate potential failures of the journal bearings, or simply to make a sensibility study of the influence of the journal bearing design parameters in the operating ones.Originality/valueThis paper provides a simple and objective methodology to make a sensibility analysis of the influence of the journal bearing design parameters in the operating parameters, as well as select the journal bearing materials and lubrication grooves.

EHD analysis of a misaligned journal bearing

Elastohydrodynamic performance characteristics of a missligned journal bearing are reported. It is observed that bush flexibility in a misaligned bearing, unlike that in an aligned bearing, improves the bearing load capacity owing to an increase in minimum film thickness at higher values of load. The misalignment is found to improve the stability of the bearing, whereas the flexibility of the bush tends to reduce this improvement.

Thermal Non-Newtonian Elastohydrodynamic Lubrication of Rolling Line Contacts

The two-dimensional thermal elastohydrodynamic equations were numerically solved for a Ree-Eyring type lubricant under pure rolling conditions. Profiles of lubricant pressure, film thickness, and temperature were obtained for medium to heavy loads and moderate to high rolling speeds. The pressure results generally show a small secondary peak near the outlet, but at the highest load considered no pressure spike is obtained and the pressure profile is almost Hertzian. The film thickness results show an increase in minimum film thickness with increasing rolling speeds, but at a lesser rate than those predicted for a Newtonian fluid under isothermal conditions. It is found that unless the lubricant becomes non-Newtonian in the inlet region, the reduction in minimum film thickness at high rolling speeds is completely due to thermal effect. The lubricant temperature profile and the amount of heat generated and dissipated in the contact region were also calculated. The lubricant temperature reaches a maximum just before the entrance to the Hertz contact region. Both shear and compression heating are found to be important in raising the lubricant temperature in the inlet. As the lubricant enters the Hertz contact zone, the temperature first drops rapidly, because of the rapid heat conduction to the rollers, and then remains almost constant for most of the Hertz contact. Near the exit where the pressure gradients are large, the lubricant temperature drops rapidly below the ambient because of lubricant expansion. The lubricant then heats up rapidly before leaving the contact area as a result of heat generated by shear stresses.