EHL Contacts Research Articles

Effect of Thermal Conductivity on Maximum Film Pressure in Point EHL Contact of Ceramics with Transverse Surface Roughness

Abstract Based on a fully flooded non-Newtonian transient thermal elastohydrodynamic lubrication analysis of representative engineering ceramics and a steel with different thermal conductivities and different surface roughness, it is clarified that material combinations leading to a reduction in the maximum film pressure can be found using the thermal conductivities of the contacting materials and the smooth surface temperatures of the same contacting materials. The results will make a significant contribution to the design of machine elements involving contact between materials of different thermal conductivity and surface roughness.

Application of artificial neural networks for the prediction of the service conditions of an elastohydrodynamic EHL contact in the presence of solid pollutant

Application of artificial neural networks for the prediction of the service conditions of an elastohydrodynamic EHL contact in the presence of solid pollutant

Pitting Influence on Electrical Capacitance in EHL Rolling Contacts

This work presents an investigation on the influence of pitting in EHL rolling contact. The pitting geometry comes as an extension on the architecture for multi-physical numerical calculations of rolling element bearing contacts based on OpenFOAM. The model utilized is built according to the state-of-the-art for fluid–solid interaction and electro-quasi-static fields. In this framework, the contact is reduced to be two dimensional in order to reduce the computing costs needed. The changes in the electric properties, which are caused by pittings, are measured using the electric capacitance of the contact and put in perspective with regard to the EHL contacts geometry. The investigation delivers an evaluation on how surface degradation, in both the pitting width w and pitting depth d of the contact, affects the electric properties. It can be seen that the maximum deviations with different reduced radii for the same pitting structures are around 5% and would therefore hardly be distinguishable with corresponding measurements. By comparing the same data for the same ratio of pitting width to reduced radius, on the other hand, differences of up to 50% can be seen.

Determination of friction coefficient for water-lubricated journal bearing considering rough surface EHL contacts

Determination of friction coefficient for water-lubricated journal bearing considering rough surface EHL contacts

A traction coefficient formula for EHL line contacts operating in the linear isothermal region

Many mechanical products including rolling/sliding parts are used with various lubricants and operating conditions. Increasing the efficiency and reliability of products requires an essential understanding of the traction characteristics of the rolling/sliding parts. Many researchers have investigated the traction characteristics of rolling/sliding EHL contacts considering shear-thinning, thermal effects, and roller compliance. There are, however, only a few papers concerning the modeling of traction characteristics in the linear isothermal region at low slide-to-roll ratios. We propose a prediction formula for the dimensionless traction coefficient for EHL line contacts in the linear isothermal region. The formula was obtained by numerical simulations using a fully-coupled finite-element EHL line contact solver, and it is applicable for the piezoviscous rigid/elastic, and the isoviscous rigid/elastic regimes.

Numerical Study on Method for Reducing Film Pressure and Its Fluctuation due to Surface Roughness in Elastohydrodynamic Lubrication Contact

Abstract One of the methods to avoid rolling contact fatigue is to reduce the film pressure due to surface roughness and the magnitude of its fluctuation. The roughness on rolling/sliding concentrated contacting surfaces has generally different asperity heights. In this study, it has been clarified, using a fully flooded non-Newtonian thermal elastohydrodynamic lubrication analysis in contact between materials with different thermal conductivities and roughness asperity heights, which surface velocity should be increased and which roughness asperity height should be lowered to reduce the magnitudes of film pressure and its fluctuation. The results obtained will make a new contribution to the combination of roughness heights on the EHL contact surfaces.

Water droplets in oil at the inlet of an EHD contact: A dual experimental and numerical investigation

The current study employs both experimental and numerical approaches to investigate water-in-oil flow at the entrance of EHD contacts. In this work, for the first time, a micro-sized water-in-oil emulsion flow was directly observed at the entrance of an EHL point contact using a high-speed camera. To represent the severity of the water-in-oil emulsion, the abscissa of the closest droplets to the contact area is compared to the critical meniscus position of starvation calculated for pure oil EHL condition. A numerical approach was developed based on the experimental observation. The impacts of various operating parameters such as rolling velocity, water droplet size, applied load, and lubricant viscosity on the flow of water-in-oil emulsion have been further investigated. The results of this work give a possible explanation for the contradictory findings in the literature and reveal the critical conditions under which water-in-oil emulsion would influence the EHL contacts.

Film Thickness in Elastohydrodynamically Lubricated Slender Elliptic Contacts: Part II – Experimental Validation and Minimum Film Thickness

In the first paper of this series, a new formula for central film thickness of slender contacts has been proposed. In this paper, interferometry measurement of the film thickness in slender elliptic contacts is performed for comparison with film thickness distribution of EHL simulation. The new film thickness formula presented in part I of this paper series is successfully validated by comparison with the experimental data. State-of-the-art minimum film thickness formulas are compared with the experimental results and results obtained through a multilevel EHL solver. Significant deviation of the analytical minimum film thickness formulas regarding both numerical and experimental results is given. Therefore, an alternative approach for calculation of minimum film thickness via the film thickness ratio with central film thickness is proposed, which yields promising results for moderately slender elliptic EHL contacts.

Numerical prediction on the effect of friction modifiers on adhesion behaviours in the wheel-rail starved EHL contact

Previous third body studies for wheel/rail are mainly concentrated on water or oil condition. However, friction modifiers (FMs) represent a new approach to managing adhesion coefficient in wheel/rail interface. In this paper, the position of the FM-air meniscus was given as input parameter, the thickness of FM can be solved easily with the multi-grid method. Non-Newtonian fluid model and micro-EHL analysis combined to obtain the relation between the adhesion coefficient and the quantity of FM. The results show that the quantity of FM has a significant effect on the initial adhesion coefficient within a certain range. Besides, the initial adhesion coefficient decreases gradually until it reach stable value with the increase of the quantity of FM.

Study on Pressure Estimation Method for Oil Film around the EHL Contact Part in Utilizing PIV

Study on Pressure Estimation Method for Oil Film around the EHL Contact Part in Utilizing PIV

Study on Measurement Method of Oil Film Thickness at EHL Contact Point in Traction Drives under High Speed by Using GMFT Method

In recent years, high-speed motors combined with traction-type reduction reducers have been considered to reduce the size and weight of electric vehicle power systems and increase their efficiency. A traction drive is a device that applies a strong load to two rolling elements and transmits power through the resulting EHL contact point. However, there is a concern that the increase in peripheral speed causes an increase in the thickness of the oil film in the contact point, resulting in a decrease in the transmission of power. In this study, the GMFT method, which enables one-shot measurement, was combined with a high-speed color camera to successfully capture the outline of the contact point under higher speed conditions than in existing studies.

Assessment of Multiaxial Fatigue Damage Criteria Based on the Critical Plane Concept in Micro-EHL Line Contact.

Micropitting is a fatigue failure phenomenon that concentrates at the surface roughness level between interacting surfaces. This type of surface fatigue is commonly recognized to exist in gears and bearings, where specific film thicknesses are sufficiently low that the rough surfaces run in the EHL condition, where the direct asperity contacts are prominent. This paper is an experimental and theoretical study to investigate a number of fatigue failure theories concerning the multi-axial fatigue models which are depended on a critical plane analysis in the mixed lubrication regime. These failure theories are namely the Findley, the Matake, the Dang Van, McDiarmid and, Fatemi and Socie model, where they are used to perform fatigue investigation for Micro- EHL contacts problem. Numerical analysis to investigate the cumulative damage and fatigue parameter in a Micro- EHL contact is established in this paper. The results of applying failure theories have indicated that the different multiaxial fatigue criteria adopted provide significant results for contact analysis in lubricated conditions, and they are more relevant to the applications of the rough surface Micro-EHL mode.

Observation of changes in meniscus around the EHL contact part by change of oil supply condition

Observation of changes in meniscus around the EHL contact part by change of oil supply condition

An imprint method to produce surface asperities for EHL and RCF experiments

A method was developed for creating single well-defined surface asperities using an imprint technique. The proposed method enables:•Creation of well-defined micrometre high asperities•Creation of asperities which survived more than 35 million EHL contact cycles•Damage tracing thanks to the possibility to control the damage initiation sites.The technique is based on rolling a hard disc with indents against a soft disc for creating single surface asperities. The contact pressure causes plastic deformation forcing material into the indents to create the asperities. The height of the asperities can be controlled by adjusting the applied force. After initial reshaping during the run-in process, the asperities were strong enough to survive more than 35 million elastohydrodynamic lubrication cycles, which should be of great interest for the researchers who investigate rolling contact fatigue experimentally. The method could also aid the research on the run-in process by enabling tracing the development of specific surface defects. Since the method can produce high and strong asperities it might also prove useful for investigations how asperities deform under sever contact conditions.

Use of Pyrene for Quantitative Fluorescence Observation of Li-Grease around EHL Contacts

Use of Pyrene for Quantitative Fluorescence Observation of Li-Grease around EHL Contacts

Transient effects of squeeze and starvation in an EHL contact under forced oscillation: On the film-forming capability

This study provides a new insight on the EHL regime in time-varying conditions. A full-analytical resolution of the Reynolds equation was proposed considering forced oscillations. Confronted to experimental validation, the analytical film thickness equations provide perfect modeling of the film forming mechanisms: squeeze induced by the transient evolution of the film thickness with time, asymmetry and hysteresis in the film distribution resulting from the change in direction and the transport effect. Furthermore, the analytical equations combined with a modulation of the inlet flow give an accurate prediction of the effects induced by the starvation resulting from the change in direction, i.e. as the original outlet zone becomes the next inlet zone.

Simplified traction prediction for highly loaded rolling/sliding EHL contacts

A simplified traction prediction approach has been developed for highly loaded EHL contacts accounting for non-Newtonian and thermal effects. For validation, a reference fluid squalane was used for traction measurements at high Hertzian contact pressures (1.25 to 2.66GPa) at three velocities of 5, 10 and 15m/s. Good agreement was seen for traction curves in a semi-log scale at a low speed of 5m/s, whereas for higher speeds at 10m/s and 15m/s, the measured traction curves are lower than the predicted ones. This reduced friction in the experimental results is caused by the increase in mass temperature of the discs. Two possible mechanisms for this reduced friction due to solid body temperature effects are discussed.

Replenishment Differences in EHL Contact Lubricated by New and In-bearings-aged Grease

Replenishment Differences in EHL Contact Lubricated by New and In-bearings-aged Grease

Investigation of the tribological performance of ionic liquids in non-conformal EHL contacts under electric field activation

Real-time external alteration of the internal properties of lubricants is highly desirable in all mechanical systems. However, fabricating a suitable and effective smart lubricant is a long-lasting experimental process. In this study, the film thickness and frictional response of ionic liquid-lubricated non-conformal contacts to an electric field excitation under elastohydrodynamic conditions were examined. Film thickness was evaluated using a “ball-on-disc” optical tribometer with an electric circuit. Friction tests were carried on a mini traction machine (MTM) tribometer with a “ball-on-disc” rotation module and an electric circuit for contact area excitation. The results demonstrate that there is a difference in the behaviour of the ionic liquid during electric field excitation at the evaluated film thicknesses. The results of evaluated film thicknesses demonstrate that there is a difference in the behaviour of the ionic liquid during electric field excitation. Therefore, the ionic liquids could be a new basis for the smart lubrication of mechanical components. Moreover, the proposed experimental approach can be used to identify electrosensitive fluids.

A study of thermal effects in EHL rheology and friction using infrared microscopy

Infrared microscopy is used to obtain through-thickness oil temperature measurements from EHL contacts between different surface materials (steel, silicon nitride and zirconia) for the lubricants Santotrac 50 and PAO4. The measurement technique was first adapted to overcome focussing issues due to the partially transparent zirconia surface. Results were used to infer in-contact rheological behaviour of the lubricants. Santotrac 50 shows significant shear localisation under all conditions with the position of the shear heating zone being highly affected by the contact surfaces' thermal properties. For PAO4, the shear profile depends on the contact surfaces’ thermal properties with moderate to high surface conductivities favouring uniform shearing, whereas highly insulating surfaces (zirconia) cause shear localisation at the surface for both lubricants. These results are used to interpret friction measurements and show how the thermal properties of surfaces can be used to control rheology and friction. This paper is prefaced by a review of thermal EHL theory upon which our analysis is based.