Lubrication Of Contacts Research Articles

Sliding friction perturbed by shear ultrasound vibrations: dynamic lubrication and overaging.

Reduction of the effective static coefficient of friction by shear ultrasound has been shown recently to be due to the partial lubrication of the solid-solid contact. Here, we study the effect of ultrasound perturbation on a multicontact interface at imposed drivevelocity. We show that, together with the partial lubrication similar to the static case, ultrasound vibrations enhance structural aging and the emergence of stick-slip.

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.

Application of Tribological Artificial Neural Networks in Machine Elements

Traditionally, analytical equations used in tribo-dynamic modelling, such as those used for predicting central film thickness within elastohydrodynamic lubricated contacts, have led to timely computations, but tend to lack the accuracy of numerical solvers. However, it can be shown that data-driven solutions, such as machine learning, can significantly improve computational efficiency of tribo-dynamic simulations of machine elements without comprising accuracy relative to the numerical solution. During this study, artificial neural networks (ANNs) are trained using data produced via numerical solutions, which are constrained by the regimes of lubrication to ensure the quality of the training data set. Multiple ANNs are then implemented to predict EHL central film thickness, as well as viscous and boundary friction, in multiple commonly used machine elements, such as a rolling element bearing and a spur gear. The viscous and boundary friction ANN prediction are compared directly against ball-on-disc experimental measurements to validate its accuracy.

A numerical strategy for predicting the interface stresses in metal strip rolling with small reduction

This paper has developed a numerical strategy for predicting the interface stresses in metal strip rolling at a small reduction. Key rolling factors such as the roll elasticity, fluid lubrication, random asperity contacts, asperity-lubricant interaction, rolling speed and reduction were comprehensively integrated into the numerical analysis. The random asperity contact at the roll-strip interface was addressed by a statistical approach with considering the elastoplastic deformations of surface asperities. A transient average Reynolds equation was utilised to deal with the fluid flow subjected to the interruptions caused by the asperity contacts. With the aid of a dynamic explicit finite element analysis, a novel modularised strategy was established to systematically accommodate the mathematical models governing the above key rolling factors, which allows the affordable computation and the effective predictions of contact characteristics in the strip rolling. It was found that both the lubrication and microscale asperity contacts disturb considerably the contact stresses at the rolling interface, and that a proper selection of rolling conditions can reasonably regulate the stresses.

Electrical stimulation enables dynamic regulation of the tribological behaviors of polyelectrolyte-modified carbon dots

Dynamic and active control of the tribological behaviors of lubricant additives under electrical stimulation is important for the development of electromechanical devices. Carbon dots (CDs) show great prospects in electrical contact lubrication owing to their easily acquired oil dispersibility, ionic conductivity, and high lubricity. Herein, the polyelectrolyte-modified CDs (PCDs) were synthesized from the chitosan-derived polyamine-terminated CDs. The PCDs can be stabilized in polyethylene glycol (PEG200) synthetic oil for more than one year without any precipitates and fluorescence decay. Compared with neat PEG200, the formulated PCDs (0.25 wt%)/PEG200 lubricant achieved 63.6% and 39.1% reductions in wear and friction, respectively. In addition, the durability of PEG200 also can be enhanced by PCDs. Crucially, the friction regulation of PCDs under external electric fields (as low as ±1.0 V) was easily realized for the first time, which effectively helps to reveal the indispensable role of polyelectrolyte shells in improving the tribological performances of PCDs. Experiments and characterizations demonstrated that the superior tribological peculiarities of PCDs benefited from the collaborations of their polyelectrolyte shells and carbon cores. The findings provide a universal protocol and material support for expediting the potential-controlled lubrication progress of nano-additives.

On roughness attenuation in elastohydrodynamic lubricated line and circular contacts

The commonly accepted “roughness attenuation” theory offers a description of the deformation of surface features as they travel through elastohydrodynamic lubricated contacts, based on the Boussinesq half-space approximation. Through a finite element model, incorporating surface features into the contact geometry (which cannot be done with the half-space approach), this work reveals three shortcomings in the theory. First, contact pressures are assumed normal to the substrate rather than the surface features lying on it. Second, it uses reduced solid properties that are invalid in the presence of surface features. Third, it does not account for the deformation of the features themselves. A correction procedure is devised, allowing for the theory to be rectified, without the need to incorporate surface features into the contact geometry, which can be computationally prohibitive.

Normal and tangential contact models for mixed lubrication of mechanical interface

Lubricating oil is usually injected in the gap between mechanical interfaces to reduce the friction and wear and improve the normal bearing capacity of the interface. The state of the mixed lubrication is induced from the inadequate lubrication. However, in the investigation of this state, the irregular oil pits of rough surfaces are usually simplified to be the cylindrical or conical in traditional contact models, which is disagreed with the actual contact situations and makes errors in the contact analysis when it is used to reveal the contact performance of the mixed lubrication. To investigate the normal and tangential contact performances for the real mixed lubrication, the normal and tangential contact models reproducing the effects of both the surface roughness and the lubrication viscosity on the normal contact force as well as the tangential fraction force are proposed in this work. Based on the statistical theory, the total area of oil pits is presented to describe irregular oil pits of rough surfaces. Then, the contact performance between the solid and liquid parts involved in the mixed lubrication is analyzed referring to the elastic–plastic theory and the hydrodynamics theory. Finally, several key influencing factors (including the surface roughness, the film thickness, and the lubrication oil viscosity) on the normal and tangential contact performances of mechanical interface are revealed. The main contribution of this work is providing some guidance on the improvement of the normal and tangential contact performances of the mechanical interface by adjusting the normal pressure, the surface roughness, the relative movement speed, and the viscosity of the lubricating medium.

Stiction and Friction of Nano- and Microtextured Liquid Silicon Rubber Surface Formed by Injection Molding

The use of cross-linking polymers such as liquid silicone rubber (LSR) can replicate serviceable surfaces with nano- and microstructures via the injection molding process. Laser ablation can be used to introduce microstructures into molding tools, while nanostructures are generated via PVD coating processes on the tools. This is why nanostructures are built using self-organized layer growth. The aim of this study was to generate evidence of direction-dependent coefficients of friction of elastomeric surfaces in dry or lubricated contact in boundary friction. Models of the dry friction of elastomeric surfaces, such as Schallamach waves or stick-slip cycles, were used to describe the friction modulation of such surfaces. Assumptions for model contacts against smooth partners, both dry and with lubrication, as well as assumptions for the interaction of structures with smooth surfaces, were investigated. It was found that for elastomer surfaces with Shore hardness 50, nanostructures are suitable for creating a direction-dependent friction increase in static and sliding friction. Friction reductions with defined microstructures are possible if their periodicity seems to interact with the wavelength of possible Schallamach waves. The choice of lubrication determines the forced wetting of the contact, but due to the structuring, there is a continuous transition to mixed friction.

Modeling of Traction Drive in Elastohydrodynamic Lubrication Contact and Traction Coefficient Enhancement Using Surface Texture Under High-Speed Conditions of up to 50,000 rpm

Abstract A high-speed electric motor with a small reducer that has high-power transmission efficiency can be used to realize a high-power-density powertrain system because electric motors can be miniaturized to increase the rotational speed. A traction drive has low vibration noise due to its lack of meshing vibration, making it suitable as a transmission element for high-speed reducers. However, the traction coefficient, which greatly affects transmission performance, decreases with increasing rotational speed. In this study, to increase the traction coefficient using a surface texture, a model that takes into account transient temperature changes under high-speed conditions and the effects of micro-surface geometry was developed. The traction coefficient was measured using a high-speed test machine capable of operating at a maximum speed of 50,000 rpm. The model was able to predict the experimental values with an error of at most 6%. The high-pressure rheological properties of the oil were examined to develop design guidelines for the surface texture and a model was used to optimize the texture parameters. The designed texture was manufactured and evaluated. Experimental results show that the traction coefficient can be improved by up to 19%.

Calculating the film thickness for the rapeseed oil in order to evaluate functioning regime

The lubricating film, generated between two surfaces in relative motion, may have theoretical thicknesses between 10-10 m and 10-5 m. This paper presents a model for calculating the minimum film thickness depending on load, sliding speed and temperature for rapeseed oil working as lubricant in a point contact and a method of designing maps of lubrication regimes for a punctual contact. The results are useful in introducing this vegetal oil in actual applications. data for viscosity-temperature dependence of the rapeseed oil are taken from literature and for the same regime parameters tests are reported on the four-ball tester. The results pointed out the sensitivity of rapeseed oil to regime parameters (load and sliding speed), but also to temperature. Calculations are done for temperatures measured in actual functioning of the four-ball tester lubricated with rapeseed oil.

Ultra-high-speed hybrid ceramic triboelectric bearing with real-time dynamic instability monitoring

The reliability and stability of bearing operation are crucial factors limiting the performance of high-speed rotating machinery. In this study, an ultra-high-speed hybrid ceramic rolling element triboelectric bearing (US-HCTEB) was developed to provide real-time dynamic behavior and stability monitoring. Herein, we adopted a floating rolling–sliding combination freestanding mode that ensures bearing structural integrity. Furthermore, a crown-shaped cage was employed with an opening, allowing the ceramic rolling element to be used as a dielectric material that was coupled with a sector-shaped interdigital electrode set on the bearing end cover to form a floating freestanding triboelectric nanogenerator (TENG). The rolling element self-rotated and revolved owing to the traction of the raceways, sweeping the electrodes to generate an alternating current. Additionally, the superiority and reliability of the US-HCTEB were demonstrated using a 30 million cycle durability test and an ultra-high rotating speed test at 16000 rpm, which far exceeds the capabilities of previously reported triboelectric bearings. The US-HCTEB output was then evaluated considering variable working conditions, structural optimization, and the surrounding environment. Dry contact lubrication and low humidity were found to be beneficial for the output performance. The spectra of the output current and the statistics describing the rolling element instantaneous speed suggest that the high-speed heavy radial load of the US-HCTEB could reduce overall skidding at the cost of increased instability. Subsequently, an extremely fast acceleration and deceleration test program was conducted on the US-HCTEB to evaluate its performance under the complex and nonstationary conditions in which high-speed bearings operate. The results revealed that the US-HCTEB not only achieved real-time and high-resolution dynamic behavior identification but also exhibited high reliability as it functioned appropriately until bearing failure. Thus, the proposed high-precision US-HCTEB can serve as an essential basis for the development of smart rolling bearings.

Effect of Transversely Isotropic Elasticity on Elastohydrodynamic Lubrication of Point Contacts.

Fiber-reinforced materials or 3D printed parts feature transversely isotropic elasticity. Although its influence on pressures, shapes, and sizes has been studied extensively for dry contacts, the transferability to lubricated contacts is fragmented. This numerical study investigates how the content and orientation of short fibers in fiber-reinforced polymers (FRP) affect elastohydrodynamic lubrication (EHL) of point contacts. Material properties are modeled with Tandon-Weng homogenization. For EHL modeling, a fully-coupled approach based on finite element discretization is used. Results on hydrodynamic pressure and film thickness as well as material stress distribution are analyzed and compared to common approximations using the effective contact moduli. It is shown that the combination of fiber content and orientation defines the effective contact stiffness that determines the contact shape, size, and film thickness. Furthermore, the contact regime can change if a contact-specific stiffness threshold is reached.

Growth and morphology of thermally assisted sinterable zirconia nanoparticle tribofilm

Tribofilm formation mechanisms of a novel zirconia (ZrO2) nanoparticle antiwear additive were investigated. Spherical ZrO2 nanoparticles 5 nm in diameter with organic capping ligands were dispersed in a polyalphaolefin (PAO) synthetic base oil and tested between AISI 52100 steel counterfaces in a ball-on-disk tribometer under boundary lubrication contact conditions. Film formation was studied as a function of temperature, using PAO blends to maintain constant viscosity across the different temperatures. A temperature dependence was found, with higher temperatures assisting initial tribofilm growth and maintaining the steady-state tribofilm thickness. Findings are consistent with previous reports indicating the primary mechanism of tribofilm growth for zirconium oxide is nanoparticle capture at the sliding interface followed by particle accumulation and sintering.

Elastohydrodynamic Lubrication of Compliant Circular Contacts near Glass-Transition Temperature

Lubrication of polymer materials nowadays represents a subject of interest in many engineering applications, such as bearings or gears, to utilize them in the areas where conventional metal materials have so far dominated. However, material properties of polymers are strongly dependent on temperature that delimits a lubrication process and leads to manifestations of viscoelastic behavior of polymers. An understanding of mechanisms, which are responsible for formation of film thickness near the glass-transition temperature, is necessary to prevent initialization of failure modes and to increase the durability of polymer engineering components. Optical chromatic interferometry was applied to investigate development of film thickness and changes in contact geometry of compliant circular contacts operated in the elastohydrodynamic lubrication regime (EHL). Film thickness was compared with soft EHL prediction models, differences in contact geometry were assessed and their contribution to film thickness development were evaluated. Qualitatively good agreement of experimental results of central film thickness and soft EHL predictions was observed; however, minimum film thickness shows significant discrepancies. Outcomes and findings confirm the operation of the compliant circular contact in Isoviscous-elastic regime of EHL and the main influence of temperature and load to thermomechanical response of amorphous polymer PMMA.

Transient Elastohydrodynamic Lubrication of Rough Contact Surface for Intermittent Motion

The current study aims to study the characteristics of transient elastohydrodynamic lubrication of smooth and rough surface contacts subjected to intermittent motion for different inactive periods of motion and deceleration/acceleration periods to fulfill an identified need for such conjunction. In this case, the non-steady Reynolds’ equation, the film thickness equation with elastic deformation and taking into account the influence of surface roughness, and the load balance equation are simultaneously solved using the Newton-Raphson with Gauss-Seidel iterations method to determine the film profile and pressure distribution at various time steps. Surface contact of sinusoidal waves is presented with different amplitudes and wavelengths. The results indicated that surface waviness causes random oscillations in pressure and film profiles at different time steps. The greater the amplitude of the waviness, the more pressure and film profile variations occur. The magnitude of the pressure and film profile fluctuations becomes little noticeable as the wavelength of a wavy surface rises. The findings of this investigation also revealed that increasing the inactive duration for wavy surfaces results in zero minimum film thickness and surface contact. Squeezing action is ineffective in conveying loading capacity in this circumstance. The central and maximum pressure heights increase during the stop time interval as stop time increases. The centre film thickness increases at the end of the deceleration phase when the deceleration/acceleration period is reduced, but the minimum film thickness is unaffected. In fact, the minimum film thickness tends to be zero at the end of the deceleration phase, regardless of the deceleration/acceleration period. The central and maximum pressure increase during the stop time interval with the decrease of the deceleration/acceleration period. The difference between the results for smooth and wavy surface contacts is the appearance of fluctuations in film and pressure profiles, as well as the reduction of the film thickness and increase in pressure for the wavy surface when compared to contact with a smooth surface.

Phospholipids and Hyaluronan: From Molecular Interactions to Nano- and Macroscale Friction

Phospholipids and hyaluronan are two key biomolecules that contribute to the excellent lubrication of articular joints. Phospholipids alone and in combination with hyaluronan have also displayed low friction forces on smooth surfaces in micro- and nanosized tribological contacts. In an effort to develop aqueous-based lubrication systems, it is highly relevant to explore if these types of molecules also are able to provide efficient lubrication of macroscopic tribological contacts involving surfaces with roughness larger than the thickness of the lubricating layer. To this end, we investigated the lubrication performance of hyaluronan, the phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and mixtures of these two components using glass surfaces in a mini-traction machine. We compared our data with those obtained using flat silica surfaces in previous atomic force microscopy studies, and we also highlighted insights on hyaluronan–phospholipid interactions gained from recent simulations. Our data demonstrate that hyaluronan alone does not provide any lubricating benefit, but DPPC alone and in mixtures with hyaluronan reduces the friction force by an order of magnitude.

Influence of Shear Effects on the Characteristics of Axisymmetric Wave Propagation in a Buried Fluid-Filled Pipe

The acoustic propagation characteristics of axisymmetric waves have been widely used in leak detection of fluid-filled pipes. The related acoustic methods and equipment are gradually coming to the market, but their theoretical research obviously lags behind the field practice, which seriously restricts the breakthrough and innovation of this technology. Based on the fully three-dimensional effect of the surrounding medium, a coupled motion equation of axisymmetric wave of buried liquid-filled pipes is derived in detail, a contact coefficient is used to express the coupling strength between surrounding medium and pipe, then, a general equation of motion was derived which contain the pipe soil lubrication contact, pipe soil compact contact and pipe in water and air. Finally, the corresponding numerical calculation model is established and solved used numerical method. The shear effects of the surrounding medium and the shear effects at the interface between surrounding medium and pipe are discussed in detail. The output indicates that the surrounding medium is to add mass to the pipe wall, but the shear effect is to add stiffness. With the consideration of the contact strength between the pipe and the medium, the additional mass and the pipe wall will resonate at a specific frequency, resulting in a significant increase in the radiation wave to the surrounding medium. The research contents have great guiding effect on the theory of acoustic wave propagation and the engineering application of leak detection technology in the buried pipe.

Wear assessment of a Ti–6Al–4V motion-preserving porous artificial-cervical-joint fabricated by SLM after surface carburization

Adjacent segment degenerative diseases are a major clinical concern for anterior cervical corpectomy with fusion surgery. In this study, a hybrid motion-preserving cervical prosthesis consists of two articulating joints and one porous fusion body is designed to provide motion ability and reduce the occurrence of degenerative diseases. The hybrid design is fabricated from Ti–6Al–4V (TC4) alloy using selective laser melting (SLM) technology. After carburization pretreatment, two types of bearing couple SLM-TC4 self-mating and SLM-TC4/UHMWPE were studied in terms of lubrication regime, contact mechanics and wear performance. According to the ISO 18192-1 standard, five million cycles (Mc) of in vitro spine simulator wear tests were conducted in bovine serum lubricated condition.Hamrock-Dawson minimum lubrication film thickness analysis stated that both bearing was likely under boundary lubrication only, with extensive surface asperities contact. And, the contact stress encountered during in vitro condition was not enough to cause surface fatigue failure of any bearing couple. In term of wear performance, the obtained steady state wear rates were 0.160 ± 0.004 mm3/Mc for SLM-TC4 self-mating and 8.57 ± 0.06 mm3/Mc for SLM-TC4/UHMWPE bearing couples, respectively. According to these outcomes, SLM-TC4 self-mating couple was more promising for the articulating joint bearing design.For SLM-TC4 self-mating couple, the initial wear mechanism was abrasion with potential third-body abrasion wear. As the wear cycle proceeding, the TiC cermet layer was gradually damaged and exposed the local bare metal. Under this circumstance, material loss was consisted of abrasion wear, adhesion wear, and tribo-corrosion. In comparison, the wear mechanism of SLM-TC4/UHMWPE bearing was always abrasion-adhesion of UHMWPE. The surface of UHMWPE socket was severely plowed by the hard micro bulge of the TiC layer, and material loss and plastic deformation occurred only on the UHMWPE socket. In conclusion, carburization pre-treatment causes a hard TiC cermet layer formation on the SLM-TC4, which enhance its hardness and protect underling material from corrosion damage. The thickness and quality of the TiC layer directly affect the longevity of the joint prosthesis in vivo.

Numerical analysis of lubrication of conformal contact with discontinuous oil droplets

Under limited lubricant supply condition, the lubrication of a slider bearing can be enhanced by discontinuous oil droplets on the lubrication track induced by poor surface wettability. This experimental finding is contrary to the general perception that strong surface wettability favors lubricating film build up. Theoretical lubrication models with the lubricant supply in the forms of oil droplets (Model Ⅰ) and uniform oil layer (Model Ⅱ) are established to clarify the odd experimental observation. Results show that oil supply in the form of droplets can achieve better lubrication performance than the uniform oil layer, which are correlated to the experimental finding. The mechanism is probably due to the early film pressure initiation when oil droplets enter the bearing contact.

Ultrathin film formation under combined effect of surface roughness and surface force for elastohydrodynamic lubrication of point contact problems

PurposeThe purpose of this study is to investigate the combined effect of surface force, solvation and Van der Waals forces and surface topography parameters of amplitude and wavelength on the formation of ultrathin films for elastohydrodynamic lubrication of point contact problems.Design/methodology/approachThe Newton–Raphson technique is used to simultaneously solve the Reynolds’ film thickness including surface roughness and elastic deformation, surface force of solvation and Van der Waals forces and load balance equations. Different values of surface amplitude and wavelength were simulated in addition to the load variation.FindingsThe simulation results revealed that roughness effects are important as the film thickness decreases. The oscillation in the pressure and film thickness is due to the combined action of the solvation force and surface topography parameters. The limiting values of the surface topography parameters of the amplitude and wavelength varied and depended on the load. For different values of wavelength and load, amplitude values up to 0.25 nm have no effect on ultrathin film formation.Originality/valueThe combined effect of the surface force and surface roughness on the formation of ultrathin films was evaluated for elastohydrodynamic lubrication of point contact problems under different operating conditions of load and surface topography parameters of amplitude and wavelength. The limited surface topography parameters of the amplitude and wavelength are shown and analyzed.