Search Results (20)

Uncontrolled wear in complex multiphysics systems can cause catastrophic failures, prompting the development of empirical methods and numerical prediction models for managing system lifetimes. This study introduces a novel approach for predicting wear on lubricated sliding surfaces by integrating rough contact mechanics into a nonlocal function with a non-uniform distribution. The model considers the sliding speed, contact area, fluid pressures, lubricant properties, and surface roughness. It employs a mixed elastohydrodynamic lubrication (mixed EHL) model to simulate lubrication and wear, using a multiscale roughness model to adjust the parameters based on the wear evolution. Validated against journal bearing data, the model accurately predicted wear rates and depths, revealing distinct roughness variations depending on the lubricant viscosity. Full article

This study presents a model of starved mixed Elastohydrodynamic Lubrication (EHL) in point and line contact to investigate the lubrication performance and material response. In formulating the governing equations for the lubrication, the dimensional Reynolds equation is discretized to involve all possible regimes from the boundary lubrication to fully hydrodynamic lubrication, and an additional algorithm is provided to determine the fractional film content based on the profiles of pressure distribution and film thickness. Solutions of the point contact from the present model are compared with those reported by the previous studies and good consistency can be found. The three-dimensional line contact is used to predict the load carrying capabilities of the film thickness at the interface of mating spur gear teeth. A return mapping method is implemented to take the plastic revolution into account. The solution at the initial stage of a startup process with the lubricant entrainment velocities of $u_x=u_y=0$ is compared with that from a dry contact to validate the elasto-plastic solutions. Sample cases are presented to reveal the effects of inlet oil supply condition and material plasticity on lubrication starvation. Full article

In the modeling of elastohydrodynamic lubrication problems considering mixed friction, strongly coupled dependencies occur due to piezo-viscous effects and asperities, which can make a numerical solution exceptionally difficult. A fully implicit coupled scheme for solving mixed elastohydrodynamic lubrication problems is presented. Our scheme uses finite-volume discretization and co-allocated grids for hydrodynamic pressure and elastic deformation. To provide strong coupling between pressure and deformation even in the highly loaded zone, a correction term that adds numerical diffusion is used. The resulting linear equation system of this scheme can be efficiently solved by Krylov subspace methods. This results in an improved accuracy and computational efficiency compared to the existing methods. This approach was validated and has been shown to be accurate. Full article

The development of high-speed motors has stimulated the demand for high-speed reducers. In response to the lack of research on high-speed reducers and the challenge of developing high-speed transmission systems, this study proposes a novel wedge-loading planetary traction drive (WPTD). First, a more accurate theoretical analysis model is established by considering the combined effects of elastic deformation, loading state, and a elastohydrodynamic lubrication (EHL) traction mechanism. Second, the mixed thermal EHL model is introduced into the performance analysis of traction drive for the first time. The fitting formulas for predicting traction contact behavior are derived, and a performance analysis method for all line-contact traction drives is presented. Third, the loading performance, transmission characteristics, and the influence of different parameters on the transmission characteristics of WPTD are analyzed. Finally, the theoretical model is validated by prototype performance tests. The results show that the loading mechanism demonstrates a good self-adaptive loading effect, and WPTD achieves a peak efficiency of 96%. Additionally, WPTD delivers superior efficiency and vibration and noise performance because of its smooth power-transfer characteristics, thereby providing a possible solution for high-speed and low-vibration transmissions. Full article

In practical scenarios, journal bearings often exhibit shape errors due to machining imperfections and operational wear. These deviations from perfect roundness can significantly impact the performance of journal bearings during start-up. This study investigates the impact of journal shape errors on transient mixed lubrications, such as water film temperature and asperity contact, as well as on the rotor dynamics of water-lubricated bearings (WLB) at start-up. The simulation results of the developed numerical model are compared with the experimental data from existing studies to verify their accuracy. Following this validation, parametric analyses are conducted using the model. The analytical results indicate that journal shape error increases the temperature rise of the water-lubricated bearing system during start-up. The greater the error in journal shape, the higher the temperature rise, both in terms of shape amplitude and waviness order. Interestingly, the thermal deformation caused by the temperature effect decreases the vertical displacement during start-up. The study also finds that higher start-up speeds lead to quicker temperature increases when shape errors are present. However, these speeds enable the bearing to more rapidly reach the elastohydrodynamic lubrication (EHL) stage, where the temperature rise stabilizes. Therefore, start-up speeds must be carefully selected. Full article

Protecting motor bearings from electric erosion is crucial as electric vehicles evolve. To better understand how lubrication interacts with electric discharge within motor bearings during varying speeds of vehicle operation, an optical ball-on-disk tribometer was modified to investigate the influence of alternating current (AC) electric fields on film thickness, friction force under various lubrication regions, and discharge characteristics. The study revealed that in AC electric fields, as the lubrication state shifts from mixed lubrication to fluid lubrication region, the electrical characteristic of the lubricating oil film changes from resistive to capacitive, accompanied by an increase in discharge frequency. Under the elastohydrodynamic lubrication (EHL) region, an electrical potential difference between the surfaces separated by the lubrication film leads to a reduction in film thickness, which can be attributed to the generation of Joule heating. If the potential difference across the oil film increases to the threshold voltage, destructive discharge occurs with the emission of a significant amount of purple light. Joule heating generated by the AC electric fields also results in a reduction in the friction coefficient under the fluid lubrication region. However, due to the reduction in film thickness, the lubrication state eventually moves to mixed lubrication, leading to a substantial increase in the friction coefficient. In addition, the study also investigated the use of grease with a nanographite conductive additive. It was found that inappropriate additive amounts can lead to discharge phenomena occurring outside the contact region. Full article

In this article, we focus on utilising electrical impedance spectroscopy (EIS) for the assessment of global and contact impedances in roller bearings. Our primary objective is to establish a quantitative prediction of lubricant film thickness in elasto-hydrodynamic lubrication (EHL) and investigate the impedance transition from ohmic to capacitive behaviour as the system shifts from boundary lubrication to EHL. To achieve this, we conduct measurements of electrical impedance, bearing and oil temperature, and frictional torque in a cylindrical roller thrust bearing (CRTB) subjected to pure axial loading across various rotational speeds and supply oil temperatures. The measured impedance data is analysed and translated into a quantitative measure of lubricant film thickness within the contacts using the impedance-based and capacitance-based methods. For EHL, we observe that the measured capacitance of the EHL contact deviates from the theoretical value based on a Hertzian contact shape by a factor ranging from 3 to 11, depending on rotational speed, load, and temperature. The translation of complex impedance values to film thickness, employing the impedance and capacitance method, is then compared with the analytically estimated film thickness using the Moes correlation, corrected for inlet shear heating effects. This comparison demonstrates a robust agreement within 2% for EHL film thickness measurement. Monitoring the bearing resistance and capacitance via EIS across rotational speeds clearly shows the transition from boundary to mixed lubrication as well as the transition from mixed lubrication to EHL. Finally, we have observed that monitoring the electrical impedance appears to have the potential to perform the run-in of bearings in a controlled way. Full article

The influence of NBR Shore hardness and PTFE elastic modulus on the sealing characteristics of step rod seals is analyzed in this paper based on the developed mixed elastohydrodynamic lubrication (EHL) model. The optimized selection studies of NBR Shore hardness and PTFE elastic modulus under different operating conditions are carried out based on the principle of minimizing net leakage and friction power loss. Results show that the Shore hardness of the NBR O-ring and, in particular, the elastic modulus of the PTFE ring has a significant effect on the sealing characteristics. Although the high values of these parameters result in high friction forces, they are beneficial for leakage control. To achieve both low leakage and low friction, it is recommended that high hardness and low modulus are selected for moderate-low pressure or high speed conditions, but low hardness and high modulus are selected for high pressure or low speed conditions. Full article

The investigation in this article focuses on the rolling resistance torque and thermal inlet shear factor in tapered roller bearings (TRBs) through systematic experiments using a modular test setup. TRBs typically operate under Elastohydrodynamic Lubrication (EHL) conditions. At sufficiently high speeds, the majority of rolling friction is due to a significant shift of the pressure centre in the EHL contact. While at lower speeds, sliding friction in the roller-rib contact becomes dominant, which operates under mixed lubrication conditions. Limited literature exists on the impact of inlet shear heating on effective lubricant temperature ($T_{in\_c}$) and rolling friction in TRBs. To fill this gap, experimental measurements of the total frictional torque under axial loading at different speeds and oil temperatures are performed. With existing models for different friction contributions described in the literature, the rolling resistance due to EHL has been determined for various operating conditions. The effects of dimension-less speed (U), material (G), and load (W) parameters have also been investigated. Under fully flooded conditions, it was observed that the influence of material (G) and load (W) parameters on rolling friction is minor, while the impact of velocity (U) is significant. In the context of rolling resistance, the heating due to shear of the lubricant in the inlet zone plays a significant role. For higher rotational velocities, the estimated rotational torque reduction resulting from inlet shear heating was found to be approximately 6–8%. Full article

Tooth surface wear is one of the most common failure modes of harmonic gears, especially in space drive mechanisms. Due to difficulty accurately modeling its wear failure model and the complex mechanism, its dynamic behavior and wear mechanism have not been deeply investigated, and study of the double-arc tooth profile wear model is relative lacking. Therefore, an improved wear modelling and analysis method that is more in line with actual conditions for double-arc harmonic gears is here proposed. Firstly, a tooth surface wear model under mixed elastohydrodynamic lubrication (EHL) was established based on the Archard formula, which combines the Reynolds equation and double-arc tooth profile equation, and considering the meshing offset caused by elastic deformation. Then, the wear analysis method combined with mixed EHL was derived, and numerical simulation analysis of the wear characteristics in lubrication state was carried out, including wear depth calculation and wear output comparison of different tooth profiles. Furthermore, the influence of main working parameters and design parameters on the wear quantity was analyzed. The results show that wear depth for mixed EHL is significantly less than at dry contact. The double-arc tooth profile can withstand more wear cycles than the involute tooth profile, and the input torque and the number of cycles significantly affect the amount of tooth wear. This study further reveals the tooth wear mechanism for harmonic gears, and provides a theoretical basis for the structural optimization design, wear reduction, and life prolonging of harmonic gears. Full article

For transient mixed Elastohydrodynamic lubrication (EHL) problems, a novel solution is required to predict friction loss and wear in sliding or rolling parts. However, existing solutions have numerous limitations. In general, the lower the oil film thickness is, the more serious the non-linear problem is. This paper presents an efficient solution to tackle the non-linearity of the mixed EHL problem. The elastic deformation in the fluid–solid iteration coupling problem is divided into two parts: One is induced by the hydrodynamic pressure. This part of the deformation is obtained by the unsteady EHL-FBNS (Fischer–Burmeister–Newton–Schur) solver by considering both mass-conserving cavitation and elastic deformation. The other part of the deformation is introduced by the asperity contact pressure. It can be obtained by the Newton–Raphson method. After some limited iterations, the mixed EHL problems can be solved by evaluating the residual total pressure (including hydrodynamic pressure and asperity contact pressure). The proposed methodology was validated against the results from the published literature and applied to characterize the tribological performance of point contact with moving texturing. It appears that the developed method can be effectively used for tracking the tribological behavior of friction pairs. Full article

Low-loss gears are an interesting design approach for increasing the efficiency and thermal load limits of gearboxes. The loss-optimized gear geometry concentrates sliding around the pitch point, which results in low load-dependent gear power losses. In this study, a method for modeling transient EHL (elastohydrodynamically lubricated) contacts in gear mesh considering mixed lubrication and thermal effects is introduced and applied to analyze the tribological behavior of a low-loss gear geometry. Special focus is placed on local frictional losses to analyze the role of the thermal effects of the gear mesh. Although a thermal reduction in fluid friction is observed, the overall effect on total frictional losses of the low-loss gear geometry is evaluated to be very small. The edge geometry strongly influences the lubricant film thickness and frictional power losses. Full article

The Reynolds equation defines the lubrication flow between the smooth contacting parts. However, it is questionable that the equation can accurately anticipate pressure behavior involving undeformed solid asperity interactions that can occur under severe operating conditions. Perhaps, the mathematical model is inaccurate and incomplete, or some HL (hydrodynamic lubrication) and EHL (elastohydrodynamic lubrication) assumptions are invalid in the mixed lubrication region. In addition, the asperity contact boundary conditions may not have been properly defined to address the issue. Such a situation motivated the recent study of a 3D CFD investigation of Reynolds flow around the solid obstacle modelled in between the converging wedge. The produced results have been compared to analytical and numerical results obtained by employing the Reynolds equation. The validated CFD simulation is compared with the identical wedge, with cylindrical asperity at the center. A significant increase in pressure has been predicted because of asperity contact. The current study shows that the mathematical formulation of the ML problem has shortcomings. This necessitates the development of a new model that can also include fluid flow around asperity contacts for the accurate prediction of generated pressure. Consequently, sustainable tribological solutions for extreme loading conditions can be devised to improve efficiency and component performance. Full article

Multi-lip reciprocating seals are extensively used in the subject of engineering equipment. However, the current research on the tribology of reciprocating seals mainly focused focuses on the numerical analysis of single-lip seals. In order to study the sealing performance of multi-lip seals, this paper takes double-acting seal (DAS) as the research object, and establishes a multi-lip mixed elastohydrodynamic lubrication (M-EHL) numerical simulation model from the perspective of solid mechanics, fluid mechanics, contact mechanics, and deformation mechanics. The sealing characteristics under different working conditions (sealed pressure, piston rod extension speed, and seal surface roughness) were analyzed by numerical calculation, and the variation trend of friction force corresponding to the experimental results was obtained. The model can provide a modeling guidance basis for the M-EHL characteristic analysis and structural optimization design of multi-lip reciprocating seals in the mechanical field. Full article

Experimental results have confirmed that parallel rough surfaces can be separated by a full fluid film. However, such a lift-off effect is not expected by the traditional Reynolds theory. This paper proposes a deterministic mixed lubrication model to understand the mechanism of the lift-off effect. The proposed model considered the interaction between asperities and the micro-elastohydrodynamic lubrication (micro-EHL) at asperities within parallel rough surfaces for the first time. The proposed model is verified by predicting the measured Stribeck curve taken from literature and experiments conducted in this work. The simulation results highlight that the micro-EHL effect at the asperity scale is critical in building load-carrying capacity between parallel rough surfaces. Finally, the drawbacks of the proposed model are addressed and the directions of future research are pointed out. Full article