Mixed EHL Research Articles

Initiation and evolution of interference wear and pitting for spur gears considering the concurrent effects of meshing impact and mixed lubrication

Extended tooth contact (ETC) leads to specific load paths, introduces periodic overload and lubrication degeneration. These disadvantages cause specific interference wear and pitting. This work proposes a wear-pitting prediction model incorporating ETC, dynamic motion, and mixed EHL to investigate coupling relations between these factors. The model predicts a MI-induced wear-pitting competition phenomenon, indicating that when the film load capacity is improved, an additional pitting streak emerges at 1.2 mm from the pinion tooth tip while the maximum wear depth decreases from 32.9 µm to 10.2 µm. Two endurance experiments under different working conditions are performed on the FZG test rig. The predicted wear distribution and pitting areas are consistent with the tested results.

A fully coupled finite element solution for the mixed elastohydrodynamic lubrication of finite line contacts

A fully coupled FE model for mixed EHL of finite line contacts is proposed, integrating all of the governing equations into a monolithic system. The solution reveals the fully coupled model achieves 5% higher accuracy and 50% greater efficiency compared to the weakly coupled one. Additionally, there is evidence to support the assertion that profiling contributes to the mitigation of surface roughness contact. Logarithmic profiling can increase the minimum film thickness by a factor of 30, resulting in a reduction of up to 64.6% in the maximum asperity contact pressure, and thus, the boundary friction force is decreased by more than 19%, under heavy-load low-velocity condition. This contributes to the reduction of abrasive wear.

Dynamic performances of novel misaligned non-uniform distributed tilting-pad bearing

The non-uniform distributed tilting-pad journal bearings (NDTPJB) in large wind turbine main shafting has become an important development trend for high-power wind turbines due to the advantages of improving reliability and reducing maintenance costs. However, studies involving NDTPJB have not been reported under specific conditions such as low speed, wide range variable direction and magnitude of heavy-load in wind turbine. A new transient mixed elastohydrodynamic lubrication (mixed EHL) model for NDTPJB is established, taking into account the misalignment effect of the shaft tilt and the swinging of pads in the circumferential and axial directions, as well as the influence of pivot friction and elastic deformation effects. Based on the mathematical model, the dynamic characteristics and structural parameter effects of NDTPJB are systematically studied under low speed, wide range variable direction and magnitude of heavy-load. The results indicate that the NDTPJB exhibits a more significant sensitivity to the heavy load direction due to the different functional roles of the above distributed pads (ADP) and bottom distributed pads (BDP). The modified mixed EHL model can more accurately evaluate the lubrication performance of NDTPJB, and combine the characteristics in individual pad swing and the overall effect of the pads to comprehensively analyze the dynamic lubrication behavior of NDTPJB. By setting the pivot coefficient to 0.55, reducing the distribution angle of BDP, and increasing the distribution angle of ADP, a better load-carrying capacity can be achieved. The research results have guiding significance for further monitoring and analyzing the experimental performance of NDTPJB, as well as designing and optimizing such bearing structures.

Investigation of friction and scuffing during loss of lubrication on a high velocity twin-disc test rig

Increasing demands on gear components such as high-pressure and high speeds are aggravated by weight saving strategies eliminating an optional secondary lubrication system during negative g operations or windmilling, which result in loss of lubrication conditions (LOL) and scuffing. In order to understand gear design options able to withstand LOL at high velocities, systematic, fundamental studies on model test-rigs, e.g. twin-disc rigs, are required. For this purpose, a twin-disc rig was designed which is able to perform test at high entrainment velocities of up to 80 m/s and high lubricant injection temperatures. Systematic studies have been carried out in continuous lubrication and LOL condition and evaluated in terms of friction and time-of failure (TOF). A mixed EHL model complemented the experimental matrix for wider range of temperatures and additionally compared the influence of the grinding direction on the lubricant gap.Entrainment velocity showed to have the most prominent influence on friction in continuous mode, mostly for the increase from 8 m/s to 23 m/s, followed by minor changes upon further increase to 30 m/s. The influence of injection temperature was mostly prominent at high entrainment velocities. Contact pressure had the strongest influence on TOF. Comparisons with superfinished disc surfaces exhibited a high increase in TOF and significantly less scuffing damage fractures on the runway.

Predictions of friction and temperature at marine cam-tappet interface based on mixed lubrication analysis with real surface roughness

The working conditions of cam-tappet pairs in marine diesel engines are directly influenced by the engine output power, the operational speed, the temperature, as well as the components surface micro-morphology, etc., which cause the cam-tappet pairs work in the mixed lubrication state, thus the interfacial friction, pressure and temperature rise are vital to engine performance, efficiency, and durability. An interfacial friction–temperature prediction model for the cam-tappet pairs, considering the effects of transient working conditions and the real surface roughness, is developed in the present study, based on the theories of the transient mixed EHL and the heat transfer under the condition of a fast moving heat source. The numerical results of surface temperature rise are compared with those from the Blok formula, and a good agreement is found. The obtained results show that the presence of 3D roughness may lead to a decrease in the lubricant film thickness, and the surface temperature rise of tappet may exceed 700 K, which is close to the material scuffing temperature, causing the surface failure due to scuffing and wear. If increasing the cam speed and base circle radius within certain range, it may lead to the increment of film thickness and reduction of surface temperature rise, thus the lubrication effectiveness is increased. In addition, using cast aluminum bronze may significantly reduce the surface temperature rise and improve the interfacial characteristics.

Finite element modeling and simulation of mixed elastohydrodynamic lubrication for the finite line contact under tilting loads

Misalignment is a prevalent problem in rotor machinery. This paper proposes a numerical analysis for the mixed EHL of finite-line contacts considering tilting loads based on FEM. The Reynolds and linear elasticity equations, and Greenwood-Tripp contact model are coupled in an iterative process to solve the fluid-solid/solid-solid interaction under the compound external force and moment. Lubrication characteristics at both ends of the cylinder are analyzed and an asymmetric profile design scheme is presented to improve the lubrication performance at the cylinder's lower end. It is found that proper profile parameters can reduce EHL pressure by 40%, increasing the minimum oil film thickness by up to 100%, and thus reduce asperity contact pressure by up to 50%.

A Curve-fit Traction Coefficient Relation of Mixed EHL Line Contact with Hydraulic Fluid and Steel Surfaces

Accurate modeling of hydraulic gear machine meshing torque loss, considering possible asperity contact, is required for better prediction of its hydromechanical efficiency. A model for predicting the mixed thermal EHL line contact traction coefficient is implemented in this study, considering experimental density and viscosity properties. Simulation results of traction coefficient are validated with experiments at two operating conditions. The defining components of the traction coefficient, the asperity load ratio and the hydrodynamic component are obtained at wide range of inputs. Finally, explicit curve-fit formulations of the asperity load ratio and the hydrodynamic component of traction coefficient are created in terms of input non-dimensional variables, which can be used for modeling the torque loss in a simple yet accurate way.

Study on the lubrication state and pitting damage of spur gear using a 3D mixed EHL model with fractal surface roughness

Study on the lubrication state and pitting damage of spur gear using a 3D mixed EHL model with fractal surface roughness

A Mixed EHL Analysis Method for Grease and Formulas for Film Thickness and Asperity Load

A Mixed EHL Analysis Method for Grease and Formulas for Film Thickness and Asperity Load

A solution for mixed elastohydrodynamic lubrication modeling considering effects of solid particles and surface roughness

To investigate the effects of solid particles on the point contact mixed lubrication, this paper aims to introduce solid particles into rough interfaces under mixed hydrodynamic lubrication. Statistical three-body micro-contact is utilized in modeling mixed EHL. The model is verified by comparing results without considering particles and roughness with those from classical EHL formula. Factors, such as particle physical parameters, surface roughness and operating condition acting on the system are discussed. The results show that the distribution of hydrodynamic pressure and oil film thickness on the point contact with particles still show typical characteristics of elastohydrodynamic lubrication. The physical parameters of particles and operating condition change the proportion of each part to the load carrying capacity, where an increase in particle size, concentration and surface orientation parameters increases the load ratio of particle and conversely an increase in surface roughness and sliding velocity decreases it. Both an increase in particle size and surface roughness result in an increase in the minimum oil film thickness, with the effect of surface roughness being more significant. In addition, the shear strength, size and concentration of the particles play an important role in the friction characteristics.

Crack nucleation and propagation modeling for lubricated spur gear contacts of rough surfaces

In this study, a rolling contact fatigue model for lubricated gear contacts of rough surfaces is proposed, considering both crack nucleation and propagation stages. A boundary element approach is implemented to describe surface roughness topography effect on near surface stress concentration, with surface tractions yielded from mixed EHL analysis. A fatigue criterion is employed to find crack nucleation position and life. Assuming mode II crack opening mechanism, stress intensity factor range is determined and used to arrive crack propagation life. Through comparisons to experiments, model capability is demonstrated. Simulation results show important roughness effect on crack propagation as well as crack nucleation.

Numerical simulation of defective gear transmission under mixed EHL conditions

Pitting is a common phenomenon for gears. The mechanisms governing the lubrication and vibration caused by the peeling of the material are not well understood, which limits the development of precision and reliability of gear transmission. In this paper, a transient mixed lubrication model is proposed to study the meshing behaviors of spur gears with pit defects. For a start, the time-varying contact parameters are calculated using tooth contact analysis along the meshing path. Then, the entire meshing process of a single teeth pair is simulated using the grid moving method. The influences of different pit geometries on gear meshing are studied, and the impacts of the pit at different speeds are also investigated. Simulation results show that defects will not only increase the asperity contact ratio but also cause fluctuations in contact stiffness and normal distance. The expansion of defect dimensions greatly affects the contact stiffness and normal distance, while the expansion of defect depth has a limited effect on them when the defect has already reached a certain depth.

Dynamical wear prediction along meshing path in mixed lubrication of spiral bevel gears

Surfaces of gears under combined rolling and sliding motions may suffer a complicated wear process due to the transient time-varying effect along the meshing path. In this paper, a methodology for predicting the wear of tooth surfaces is developed for the spiral bevel gears. In the wear model, the machined surface roughness, mixed lubrication, friction, flash temperature and the dynamic behavior of gears are all considered. Tooth-Contact-Analysis (TCA) method is used to get the time-varying parameters of meshing points along the meshing path. By simulating real movement process, the material is removed according to the Arrhenius equation. First, the distribution of pressure and film thickness is obtained by solving the mixed EHL model. After that, the flash temperature can be computed by the point heat source integration method with the obtained pressure, film thickness and velocity vector. The material removal is based on surface temperature and sliding distance. The numerical results are compared to the ball-on-disk experiments to demonstrate the reasonableness of the present wear model. And it shows that the angle difference between velocity vectors has strong influences on the wear profile. Furthermore, the mechanism of surface wear evolution is investigated systematically in spiral bevel gears. The difference of the wear track between the pinion and gear surfaces is observed. Besides, in the meshing process of tooth surface, the wear along the meshing path is uneven, which appears to be much greater at the engaging-in and engaging-out areas. There is a position with maximum wear rate in the meshing process, and the position is affected by the load and speed.

On the Relation between Friction Increase and Grease Thickener Entraining on a Border of Mixed EHL Lubrication

This paper deals with an experimental study of film thickness and friction of commercial-grade grease and its base oil in a highly loaded contact. In-situ measurements were conducted for two surface textures on a ball-on-disc optical tribometer at the border of mixed lubrication. At high speeds, the film thickness and the friction of grease correspond with the base oil, while, the thickener enters the contact area and locally affects the film thickness and friction at low speeds. It was found out that the thickener starts to enter the contact area approximately at the same speed when the base oil friction increases on Stribeck curve but without direct solid to solid contact. It indicates that both effects can have the same origin. Change of lubricant flow in contact inlet area was discussed as a possible explanation.

Improved mixed elastohydrodynamic lubrication of hypoid gears by the optimization of manufacture parameters

Extensive wear appears in the case of dry contacts, or when the lubrication of the contacting surfaces is not appropriate. The aim of this research is to improve the mixed elastohydrodynamic lubrication in hypoid gears by the optimization of manufacture parameters for tooth surface processing.A full numerical analysis of the thermal mixed EHL in hypoid gears is applied. The equation system and the numerical procedure are unified for a full coverage of all the lubrication regions including the full film, mixed, and boundary lubrication. In the hydrodynamically lubricated areas the calculation method employed is based on the simultaneous solution of the Reynolds, elasticity, energy, and Laplace's equations. In the asperity contact areas the Reynolds equation is reduced to an expression equivalent to the mathematical description of dry contact problem. The real geometry and kinematics of the gear pair based on the manufacturing procedure is applied, thus the exact geometrical separation of the mating tooth surfaces is included in the oil film shape, and the real velocities of these surfaces are used in the Reynolds and energy equations. The transient nature of gear tooth mesh is included. The oil viscosity variation with respect to pressure and temperature and the density variation with respect to pressure are included. The non-Newtonian behaviour of the lubricant is considered. Using this model, the pressures, film thickness, temperatures, and power losses in the mixed lubrication regime are predicted. By using the developed method, the influence of the manufacturing parameters on the conditions of mixed elastohydrodynamic lubrication is investigated. On the basis of the obtained results recommendations are formulated to improve the mixed EHL and the efficiency of face-milled hypoid gears.

Transient behaviors of friction, temperature and fatigue in different contact trajectories for spiral bevel gears

Abstract The dramatic changes in contact geometry, load, entraining and sliding velocity vectors along contact trajectories of spiral bevel gears may cause significant squeezing action. It suggests that the steady-state analysis at each discrete engaging point cannot effectively describe the tribological behavior in spiral bevel gears. The present study aims to systematically study the transient friction, temperature and contact fatigue behaviors in different contact trajectories for spiral bevel gears based on a newly developed transient mixed EHL model. It shows that the obtained transient friction coefficient is much higher than the steady-state when the gears mesh in the first half of the engagement period. During the entire engagement period, the predicted transient fatigue life is significantly longer than the predicted steady-state fatigue life, which is of importance to the integrated and economical design of spiral bevel gears. The comparison with single half-sine waveform asperity demonstrated that the transient asperity contact pressure is much lower than that in quasi-steady state. The variation of flash temperature appears to be insignificant. Moreover, in different contact trajectories, the weakest point in one engagement period always occurs at a certain point when one teeth pair just finished the engaging-in and another pair of teeth completed engaging-out, and the toe contact exhibits the maximum frictional energy consumption and shortest fatigue life among them.

Improvements in the mixed elastohydrodynamic lubrication and in the efficiency of hypoid gears

In this paper, the influence of the manufacturing parameters on the conditions of mixed elastohydrodynamic lubrication is investigated. On the basis of the obtained results, recommendations are formulated to improve the mixed EHL and the efficiency of face-milled hypoid gears. A full numerical analysis of the mixed EHL in hypoid gears is applied. The equation system and the numerical procedure are unified for a full coverage of all the lubrication regions, including the full film, mixed, and boundary lubrication. In the hydrodynamically lubricated areas, the calculation method employed is based on the simultaneous solution of the Reynolds, elasticity, energy, and Laplace's equations. In the asperity contact areas, the Reynolds equation is reduced to an expression equivalent to the mathematical description of dry contact problem. The real geometry and kinematics of the gear pair based on the manufacturing procedure are applied; thus, the exact geometrical separation of the mating tooth surfaces is included in the oil film shape, and the real velocities of these surfaces are used in the Reynolds and energy equations. The transient nature of gear tooth mesh is included. The oil viscosity variation with respect to pressure and temperature and the density variation with respect to pressure are included. The non-Newtonian behaviour of the lubricant is considered. Using this model, the pressures, film thickness, temperatures, and power losses in the mixed lubrication regime are predicted. The effectiveness of the presented method is demonstrated by using hypoid gear examples.

Mixed EHL analysis of planetary drives with small teeth number difference considering real tooth geometry

AbstractPlanetary drives with small teeth number difference are key components widely used for transmitting significant power. In general, these components are designed with short tooth depth to avoid the interference of addendum, which may have significant effect on contact conditions and lubrication characteristics. In the present study, a modified mixed elastohydrodynamic lubrication model is employed to analyse the lubrication performance in tooth pairs with consideration of real geometry profile (short tooth depth) and 3D machined roughness. Numerous sets of cases from meshing in to meshing out have been analysed under a wide range of operating conditions to study the tribological behaviour. Obtained results suggest that the lubrication and friction behaviours are much worse especially at meshing out points than those ignoring the influence of real tooth geometry. The modified elastohydrodynamic lubrication model is compared with finite element analysis results which indicate that it is able to handle practical cases in which the real contact width is much shorter than Hertzian contact width and can be employed for the analysis and design of planetary drives with small teeth number difference.

A Mixed EHL Model of Finite-Length Line Contact Considering Asperity Elasto-Plastic Deformation and the Lubricant Properties

A Mixed EHL Model of Finite-Length Line Contact Considering Asperity Elasto-Plastic Deformation and the Lubricant Properties

Effect of contact path on the mixed lubrication performance, friction and contact fatigue in spiral bevel gears

Abstract The contact path in spiral bevel gears may have significant effect on the lubrication breakdown and surface failure. However, available study mainly focus on the influence of contact path on the contact stress distribution with the assumptions of dry contact and smooth surface. In reality, these gears are lubricated and the surfaces are quite rough with three-dimensional (3D) topography. The present study aims to systematically investigate the effect of contact path on the mixed lubrication characteristics, friction and contact fatigue in spiral bevel gears with the consideration of surface roughness. A comprehensive analysis for gearing geometry, kinematics, and contact load with different contact path affected by assembling parameters is executed based on the loaded tooth contact analysis (LTCA). The corresponding lubrication characteristics, friction coefficient and flash temperature, as well as the fatigue life are conducted in a wide range of operating condition based on a recently developed mixed EHL model for spiral bevel gears. Obtained results reveal that the contact path is vital to lubrication performance, efficiency improvements and fatigue prevention. In particular, the contact path nearby toe of gear flank appears to be a weak path due to the smaller film thickness, larger friction coefficient, higher flash temperature and lower relative fatigue life.