•A fractal thermal contact conductance prediction model considering thermal stress and asperity interactions is proposed.•The effects of contact load, surface topography, temperature difference and ...material properties on TCC are discussed in detail.•TCC obtained from the proposed model is closer to experimental results compared to the relatively reported models at intermediate and large contact pressure.
The thermal contact conductance (TCC) of rough surfaces is a fundamental issue in heat transfer. Thermal stress and asperity interactions have important impacts on the TCC. A new fractal model for predicting the TCC that considers thermal stress and asperity interactions is developed. First, an improved normal contact mechanics model is constructed that considers the asperity deformation, thermal stress, and interactions of a single asperity from a microscopic viewpoint. Then, a new TCC prediction model is proposed according to the improved contact mechanics model and classical heat conduction theory. Furthermore, the predicted values of the TCC are compared with published experimental results and reported models. Finally, the influences of surface topography, temperature differences, and material properties on the TCC are further revealed. This study can provide deep insight into the thermal design of sophisticated equipment.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Most mechanical systems have mating interfaces that require effective lubrication for smooth functioning. Thus, understanding and accurately modeling lubrication regimes is an essential research ...task. The present work proposes a set of normalized coefficients based on homogenization and stochastic theories to model the partial and full-film lubrication regimes between a smooth and a rough surface. The modified Greenwood and Williamson theory, considering inter-asperity-interactions, was used to model the contact-zones. Subsequently, the film-thickness was evaluated and the homogenized Reynolds equations were solved to compute the normalized coefficients. The estimated coefficients were compared with Patir and Cheng’s flow-factors, and the similarities and disagreements between both approaches were discussed. New sets of coefficients for running-in surfaces were proposed for modeling their lubrication regimes.
•Normalized homogenization coefficients are evaluated based on stochastic analysis.•Inter-asperity interactions have been considered to model the solid contacts.•New sets of homogenization coefficients for running-in surfaces are proposed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZRSKP
Gears typically operate in mixed lubrication conditions, where the lubricant film is too thin to prevent opposing surface asperities from interacting with each other. The likelihood/intensity of ...interactions is indicated by the
ratio: the ratio of smooth surface film thickness to surface roughness. Researchers have asserted that asperity interactions are the predominant cause of acoustic emission (AE) in healthy gear contacts. However, direct experiments on gears have yet to yield a clear relationship between the Asperity AE (AAE) and
ratio, this is in part due to the complexity of gear tooth contacts. In this paper, a disc rig was used to simulate a simplified gear contact so that the fundamental relationship between AAE and
could be investigated more effectively. By varying oil temperature and entrainment speed, a wide spectrum of lubrication conditions was generated. In contrast to other published studies, an independent measurement technique, the contact voltage (CV), was used to verify the amount of interactions, and repeated roughness measurements were used to confirm minimal surface wear. A simple, consistent and precise relationship between AAE amplitude and
was identified and defined for changes from full-film to mixed lubrication. Within the mixed lubrication regime, the AAE amplitude increased exponentially as
decreased at all speeds tested. It was also observed that an increase in speed always resulted in an increase in AAE amplitude, independently of any changes in
. This direct effect of speed was modelled so that the AAE could be predicted for any combination of speed and
within the tested envelope. This paper links gear contact tribology and AE with new precision and clearly demonstrates the potential of using AAE as a sensitive monitoring technique for the lubrication condition of gears.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
If two rough surfaces slide against each other, the typical contact event is a transient interaction of a pair of asperities. A recent finite element solution of the heat conduction through such a ...contact is used to develop an expression for the heat transferred between the bodies as a function of the surface statistics, the nominal contact pressure and the sliding speed. Simple curve fits are provided to permit these results to be implemented as a macroscopic heat transfer condition in numerical simulations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The torsional dynamics of a vehicle differential hypoid gear pair is investigated. The model comprises applied torque, representing transmitted engine power, including engine order vibration. A ...number of gear teeth pairs transmit the applied torque through their lubricated conjunctions. Tooth contact analysis (TCA) is used to obtain the appropriate geometrical, kinematic and meshing parameters. These enable the evaluation of contact loads, film thickness and friction for conjugate teeth pairs, which are subject to mixed thermo-elastohydrodynamic regime of lubrication. It is shown that the lubricant undergoes non-Newtonian shear in line with the Eyring regime of traction. The inclusion of combined thermal non-Newtonian shear and boundary interactions has not hitherto been reported for the tribo-dynamics of hypoid gear pairs. When rate of change of gear teeth contact radii is included in the analysis more complex system dynamics (loss of teeth contact) result, particularly at higher speeds. The stated features constitute the main contributions of the current work, which have not hitherto been reported in literature. It is also shown that teeth contact separation ensues when resonant conditions are noted. This is regarded as the main root cause of a noise and vibration phenomenon, known as axle whine.
► We model the torsional dynamics of hypoid gears with contact radii time variation. ► Tooth Contact Analysis and thermal non-Newtonian shear/boundary effects are used. ► The teeth contact lubrication regime is thermo-elastohydrodynamic. ► The lubricant undergoes non-Newtonian shear in line with the Eyring traction regime. ► Complex system dynamics/teeth separation are found near 1:1 resonant conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Interactions between neighboring asperities play an important role in evaluating the contact status accurately during the loading process for rough surfaces. In this paper, a three-dimensional finite ...element model considering interactions between two asperities is built for electrical contact analysis. Based on the simulation, effects of contact radii of a-spots and horizontal spacing between asperities on the contact area and contact resistance are investigated explicitly. The simulation results are compared with the theoretical results ignoring interactions for the same parameters. Furthermore, the critical conditions of considering interactions between asperities are determined. In addition, curve-fit equations for dimensionless contact resistance including the influence of asperity interactions are presented, which could be directly applied to the analysis for the rough surfaces in contact.
An improved elastic micro-contact model of rough surfaces accounting for asperity interactions is proposed. The contact behavior of a single asperity system is composed of a stiffer hemi-spherical ...asperity deformation and bellowing softer substrate deformation, which is then extended to rough surface contact including asperity interactions. Using the solution of substrate deformation, normal positions of individual asperities are adjusted during quasi-static contact, from which surface interactive forces are obtained. Analytical simulations are performed using the proposed rough surface contact model, whose results are compared to Greenwood–Williamson-based models and with experimental measurements.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Discrete dislocation plasticity was used to analyse plane-strain indentation of a single-crystal elastic-plastic semi-infinite medium by a rigid surface exhibiting multi-scale roughness, ...characterised by self-affine (fractal) behaviour. Constitutive rules of dislocation emission, glide and annihilation were used to model short-range dislocation interactions. Dislocation multiplication and the development of subsurface shear stresses due to asperity microcontacts forming between a single-crystal medium and a rough surface were examined in terms of surface roughness and topography (fractal) parameters, slip-plane direction and spacing, dislocation source density, and contact load (surface interference). The effect of multi-scale interactions between asperity microcontacts on plasticity is elucidated in light of results showing the evolution of dislocation structures. Numerical solutions yield insight into plastic flow of crystalline materials in normal contact with surfaces exhibiting multi-scale roughness.
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BFBNIB, GIS, IJS, KISLJ, NUK, PNG, UL, UM, UPUK
Crack surface interference under cyclic shear loads is studied by an analytical method. The proposed model simulates the effects arising from both the residual stresses and the asperity interactions. ...A closed‐form and a discrete approach are presented in obtaining the crack surface interference solutions. Backlashes of shear displacements, peeling or group sliding behaviours and induced cyclic mode I stress intensities are predicted under three configurations of residual stress distributions. The effects of a static mode I load, the facet angle and the frictional angle are also analysed. The predicted relationship between the effective shear mode stress intensity range and the R‐ratio is discussed together with the experimentally observed ‘contrasting’ R effects.
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BFBNIB, DOBA, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UILJ, UKNU, UL, UM, UPUK
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