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Internal gearings are commonly used in transmissions due to their advantages like high-power density. To ensure high efficiency, load-carrying capacity and good noise behavior, a profound knowledge of the local gear mesh is essential. The tooth contact of internal gears relates to a convex and concave surface that form a conformal contact. This is in contrast to external gears, where two convex surfaces form a contraformal contact. This paper aims at a better understanding of conformal contacts under elastohydrodynamic lubrication (EHL) to improve the design of internal gearings.

An existing numerical EHL model is used for studying the characteristic properties of a hard conformal EHL line contact. A hard contraformal EHL line contact is studied as reference. Non-Newtonian fluid behavior and thermal effects are considered. By taking into account the local contact conformity and kinematics, the effects and relevance of the curvature of the lubricant gap and micro-slip are analyzed. In a parameter study, scale effects of the contact radii on film thickness, temperature rise and friction are examined.

The curvature of the lubricant gap and effects of micro-slip are small in hard conformal EHL line contacts. For high micro-slip, it can be neglected. Hence, the modeling of conformal contacts using an equivalent geometry of the contact problem is reasonable. The parameter study shows beneficial tribological aspects of the conformal contact compared to the contraformal contact. Higher film thickness and lower fluid coefficient of friction are observed for conformal contacts, which can be attributed to lower pressures for the case of the same external normal force, or to a higher contact temperature rise for the case of equivalent contact pressure.

Despite its widespread existence, the local geometry and kinematics in hard conformal EHL line contacts like in internal gearings have been rarely studied. The findings help for a better understanding of local contact characteristics and its relevance. The quantified scale effects help to improve the efficiency and load-carrying capacity of machine elements with hard conformal EHL contacts, like internal gearings.

The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-12-2022-0366/

Tribological research is complex and multidisciplinary, with many parameters to consider. As traditional experimentation is time-consuming and expensive due to the complexity of tribological systems, researchers tend to use quantitative and qualitative analysis to monitor critical parameters and material characterization to explain observed dependencies. In this regard, numerical modeling and simulation offers a cost-effective alternative to physical experimentation but must be validated with limited testing. This paper aims to highlight advances in numerical modeling as they relate to the field of tribology.

This study performed an in-depth literature review for the field of modeling and simulation as it relates to tribology. The authors initially looked at the application of foundational studies (e.g. Stribeck) to understand the gaps in the current knowledge set. The authors then evaluated a number of modern developments related to contact mechanics, surface roughness, tribofilm formation and fluid-film layers. In particular, it looked at key fields driving tribology models including nanoparticle research and prosthetics. The study then sought out to understand the future trends in this research field.

The field of tribology, numerical modeling has shown to be a powerful tool, which is both time- and cost-effective when compared to standard bench testing. The characterization of tribological systems of interest fundamentally stems from the lubrication regimes designated in the Stribeck curve. The prediction of tribofilm formation, film thickness variation, fluid properties, asperity contact and surface deformation as well as the continuously changing interactions between such parameters is an essential challenge for proper modeling.

This paper highlights the major numerical modeling achievements in various disciplines and discusses their efficacy, assumptions and limitations in tribology research.

The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-03-2023-0076/

The paper focuses on the solution of a numerical model to explore the sliding and non‐Newtonian fluid behaviour in soft elastohydrodynamic nip contacts. The solution required the coupling of the fluid and elastomer regimes, with the non‐Newtonian fluid properties being described using a power law relationship. The analysis showed that the fluid characteristics as defined by the power law relationship led to large differences in the film thickness and flow rate with a movement of the peak pressure within the nip contact. The viscosity coefficient, power law index and sliding ratio were shown to affect the nip performance in a non‐linear manner in terms of flow rate and film thickness. This was found to be controlled principally by the level of viscosity defined by the power law equation. The use of a speed differential to control nip pumping capacity was also explored and this was found to be most sensitive at lower entrainment speeds.

Coatings are widely used in gears to keep interface from wearing excessively. The purpose of this paper is to study the effect of coating properties and working conditions on the pressure, the shear traction, stresses as well as the fatigue life of spur gear.

A numerical contact fatigue life model of a coated spur gear pair under elastohydrodynamic lubrication (EHL) is developed based on the characteristics of gear geometry and kinematics, lubrication conditions and material properties. Frequency response functions and the discrete convolute and fast Fourier transform (DC-FFT) algorithm are applied to obtain elastic deformation and stress. Mutil-axial fatigue criteria are used to evaluate the contact fatigue life based upon the predicted time-varying stress fields of coated bodies.

The maximum Mises stress decreases while the fatigue life increases as the coating modulus decreases. A thinner coating leads to a longer life and a smaller maximum Mises stress for hard coatings. The load has more significant effect on the contact fatigue life of soft coatings.

The developed model can be used to evaluate the contact fatigue life of coated gear under EHL and help designers analyze the effect of coating elastic modulus and thickness on the contact pressure, film thickness and stress.

The main purpose of this paper is to present the effort on developing a mixed elastohydrodynamic lubrication (EHL) model to study the tribological effect of asperities on rough surface.

The model, with the use of the average flow Reynolds equation and the K-E elasto-plastic contact model, allows predictions of hydrodynamic pressure and contact pressure on the virtual rough surface, respectively. Then, the substrate elastic deformation is calculated by discrete convolution fast-Fourier transform (DC-FFT) method to modify the film thickness recursively. Afterwards, corresponding ball-on-disk tests are conducted and the validity of the model demonstrated. Moreover, the effects of asperity features, such as roughness, curvature radius and asperity pattern factor, on the tribological properties of EHL, are also discussed though plotting corresponding Stribeck curves and film thickness shapes.

It is demonstrated that the current model predicts very close data compared with corresponding experimental results. And it has the advantage of high accuracy comparing with other typical models. Furthermore, smaller roughness, bigger asperity radius and transverse rough surface pattern are found to have lower friction coefficients in mixed EHL models.

This paper contributes toward developing a mixed EHL model to investigate the effect of surface roughness, which may be helpful to better understand partial EHL.

The purpose of this study is to propose a new method to solve transient elasto-hydrodynamic lubrication (EHL) problem.

First, the steady-state EHL solution is modified so that the elastic deformation theory is combined with oil film stiffness distribution instead of steady-state Reynolds equation. Second, subsequent dynamic EHL procedure develops, recursively using transient distributed oil film stiffness and damping, where each time-marching solution is iteratively searched by ensuring both oil film force growth and elastic deformation update for each load increment.

This method increases calculation speed and provides both distributed EHL stiffness and damping for transient regimes.

This method is of interest for fast applications such as rolling bearings or gears.

The purpose of this study is to investigate the influence of surface texture on the subsurface characteristics of contact interfaces under elastohydrodynamic lubrication condition. As a typical contact form of gears and bearings, the optimization of friction characteristics at the elastohydrodynamic lubrication (EHL) interface has attracted the attention of scholars. Laser surface texturing is a feasible optimization solution, but there have been concerns about whether the surface texture of high-pair parts will affect their fatigue life.

To examine the impact of texture preparation on the subsurface characteristics of high-pair interfaces under EHL conditions, a point contact EHL model is developed that takes into account the effect of textured surface topography. The pressure and thickness of the oil film are calculated as input parameters under different loads and entrainment velocities. The finite element method is used to simulate the impact of textures with varying diameters, densities and depths on the subsurface characteristics of the elastohydrodynamic interface. According to ISO 25178, analyze the relationship between 3D topography parameters and subsurface characteristics and study the trend of friction characteristics and subsurface characteristics based on the results of the ball on disc friction tests.

The outcomes suggest that under different rotational velocity and load conditions, the textured surfaces exhibit improved friction reduction effects; however, the creation of textures can result in significant subsurface plastic deformation and local peeling. The existence of texture makes the larger stress zone in the subsurface layer closer to the surface, leading to fatigue failure near the surface. Reasonable design parameters can help enhance the attributes of the subsurface. A smaller Sa and a Str greater than 0.5 can achieve ideal subsurface properties on the textured surface.

This paper investigates the influence of surface texture on the friction and subsurface characteristics of EHL interfaces and analyzes the impact of surface texture on interface contact performance while achieving lubrication improvement functional characteristics. The results provide theoretical support for the optimization design and functional regulation of surface texture in EHL interfaces.

The peer review history for this article is https://publons.com/publon/10.1108/ILT-10-2023-0324/

This paper aims to investigate the effect of changing speed of the entraining motion on the formation of ultra-thin lubricating films under different elliptical ratios. The ellipticity parameter (K) varied from 1 (a ball-on-plate configuration) to 6 (a configuration approaching line contact). The influence of the ellipticity parameters, the dimensionless speed and the effects of surface forces on the formation of the minimum film thickness has been demonstrated. The demarcation boundary between region dominated by elastohydrodynamic lubrication (EHL) and that by the surface force action has been demonstrated for different elliptical ratios.

The numerical solution has been carried out, using the Newton–Raphson iteration technique, applied for the convergence of the hydrodynamic pressure. The film thickness and pressure distribution are obtained by simultaneous solution of the Reynolds’ equation, the elastic deformation (caused by hydrodynamic pressure, surface force of solvation and Van der Waals force) and the load balance equation. The operating conditions, load and speed of entraining motion, promote formation of ultra-thin films that are formed under the combined action of EHL, surface contact force of solvation and molecular interactions due to presence of Van der Waals force.

The paper provides insights about the transition between region dominated by EHL and that by the surface force action for changing ellipticity ratio (K) from 1 (a ball-on-plate configuration) to 6 (a configuration approaching line contact).

This paper fulfils an identified need to study the effect of changing ellipticity ratio on the formation of ultra-thin films that are formed under the combined action of EHL, surface contact force of solvation and molecular interactions due to presence of Van der Waals force.

The purpose of this paper is to study starvation in grease-lubricated finite line contacts and to understand film-forming mechanisms of grease-lubricated finite line contacts.

A multiple-contact optical elastohydrodynamic (EHL) test rig is constructed to investigate the influences of lubricant properties on film thickness and lubrication conditions at different working conditions. The film thickness is calculated according to the relative light intensity principle. The degree of starvation is evaluated by the air–oil meniscus distance and the corresponding film thickness.

The experimental results show that for greases with high-viscosity base oil, the high-frequency fluctuation of film thickness is observed in low-speed operating conditions. Reducing the viscosity of the base oil and improving running speed can weaken the fluctuation of film thickness. The degree of starvation increases with increasing base oil viscosity, rolling speed and the crown drop. In addition, reducing the replenishment time by reducing the gap between the rollers also can increase the degree of starvation.

Starvation is often to occur in finite line contacts, such as roller bearings and gears; there are still limited finite line contact EHL test rigs, much less multiple-contact optical test rigs. Therefore, the present work is undertaken to construct the multiple-contact test rig and to evaluate the mechanism of starvation in finite line contacts.

The study focuses on the development of a numerical model to explore the impact of surface roughness in soft rolling nip contacts, including representation of a real surface. The solution of the governing equations required the application of a multigridding technique to capture the details of the fluid flow within the roughness wavelengths and a minimum number of fluid nodes per wavelength were established. In the case studies, two extreme roughness profiles were considered, longitudinal and circumferential. The longitudinal roughness had a significant impact on nip pressures and pumping capacity, the latter being determined by the minimum film thickness in the nip. The circumferential roughness was found to have a localised effect on film pressure, but only a very small impact on the film thickness profile. The consequent effect on pumping capacity was small.