Modelling interfacial tribochemistry in the mixed lubrication regime

The need to reduce the cost of components is driving more and more machine elements to operate under mixed lubrication conditions. With higher operating pressures, the lubricant film is becoming thinner and eventually reaches nanometre scales, comparable to the surface roughness. Thus, understanding the mixed lubrication phenomenon is becoming increasingly important. However, the mixed lubrication phenomenon is difficult to capture experimentally and the lubricant additive ZDDP (Zinc Dialkyl Dithio Phosphate) shows its full antiwear character in the mixed lubrication conditions. This research stems from the need for models that can simulate contact mechanics, lubrication and tribochemistry in a single framework. This is the key to understanding and optimizing the lubrication systems to meet future needs. To this end, a numerically efficient procedure based upon the tridiagonal solution of the Reynolds equation is developed and is implemented, in Fortran to solve the equations line by line to incorporate more information from the current iteration step. The asperity contacts are handled by the unified solution algorithm. A new strategy to simulate plastic deformation in a lubricated contact is developed. Under practical loading conditions, the pressures inside the contact can reach values far above the material yielding limit. Thus, an efficient numerical scheme is devised to include the elastic perfectly plastic behaviour in the EHL solution procedure to simulate realistic contact conditions with minimal increase in computational cost. The Boussinesq deformation integrals result in a convolution of pressure and the deformation which is solved using Fast Fourier Transforms (FFTs) by modifying the solution domain to create a cyclic convolution. Code is developed to allow exploration of the highly optimized C-based library (www.fftw.org). The use of FFTs speeds up the solution process many times and makes the use of denser grids and larger time scales accessible. A mesh size of 129 x 129 is found to give reasonable results. The simulation results from the current study agree very well with the previously published results. The evolution of contact area ratio and the central film thickness exhibit a Stribeck type behaviour and the transition speeds at which the contact transits from EHL to mixed and from mixed to complete boundary lubrication can be precisely identified. Existing tribofilm growth models developed for boundary lubrication are reviewed and a model based on the interface thermodynamics is adapted and integrated with the mixed lubrication model to simulate tribochemistry. The problems with existing EHL concepts such as lambda ratio and central film thickness are identified and new definitions are proposed to allow understanding of the mixed lubrication mechanics. The mutual interaction between the tribofilm growth and lubricant film formation is studied. Finally the wear of the tribological system is studied and the wear track profiles are predicted. The new model is then applied to study a ball-on-disc system to explore wear, tribochemistry and roughness evolution. The ZDDP tribofilm growth is studied and the it is found that the final ZDDP tribofilm thickness is very weakly affected by increasing SRR but the rate of formation and removal are strongly affected by the SRR value. The tribofilm growth results are validated against published numerical and experimental results. It is found that the antiwear action of the ZDDP tribofilm is not only due to its chemical action but the ZDDP tribofilm helps to entrain more lubricant and improves contact performance. The presence of tribofilm roughens the contact and the contact area and load ratio both increase due to tribofilm growth. It was also found that the use of conventional EHL parameters to analyse the behaviour of tribosystem is misleading. The flattening of the roughness inside the contact and the proper identification of the central film thickness are crucial to the interpretation of the mixed lubrication results. The roughness of the ball generally decreases due to wear but the presence of tribofilm limits this reduction. Contrary to this, the surface roughness of the ball generally increases due to wear but the presence of tribofilm results in increased roughness of the ball

The mutual interaction between tribochemistry and lubrication: Interfacial mechanics of tribofilm

A new mechanism for the action of antiwear tribofilms is proposed. The antiwear action of ZDDP additive is believed to be mainly due to the formation of tribofilms that reduce wear by chemical action. In this study, a mixed lubrication model is developed and tribofilm growth integrated into this model to simulate the effects of tribofilms on lubrication. The dynamic evolution of the contacting surfaces due to plastic deformation, wear and tribofilm growth continuously change the lubrication characteristics inside the contact. It is observed that the growth of tribofilm roughens the contact and increase contact severity. It was found that this roughness increase also helps to entrain more lubricant, resulting in thicker lubricant films. Therefore, the plot of the evolution of film thickness ratio (hcentral(t)/Rq(t)) shows that the lubrication regime is improved by the presence of tribofilm. Therefore, not only the chemical presence but the physical presence of the tribofilm on the surfaces also helps to improve contact performance by retaining more lubricant and improving the lubrication regime

A simple deterministic plastoelastohydrodynamic lubrication (PEHL) model in mixed lubrication

Most power transmitting components operate under mixed lubrication conditions. Concentrated pressures and smaller lubricant film thickness may cause surface and subsurface stresses to exceed the material yield limit causing permanent geometrical changes. A model was developed to include elastoplastic behaviour within a deterministic unified mixed lubrication framework. Model details are presented and the model is validated against published simulation data. A parametric study to address the effect of material yielding on the contact parameters is performed. It is found that the model successfully produces all the key features of the PEHL contact. The model provides a valuable tool to analyse the PEHL contacts with minimal increase in computational effort and complexity

EXPERIMENTAL BENCHMARKING OF SURFACE TEXTURED LIP SEAL MODELS

A thorough investigation on the existing hydrodynamic lubrication theories and the reverse pumping theories for the conventional lip seal is conducted. On that basis, the algorithms and the methods used in the numerical modeling of the conventional lip seal are modified and applied to the study of the lip seal running against surface textured shafts. For each step of the study, the numerical model is benchmarked against the experimental results. Important physical mechanisms which explain the reverse pumping ability of the triangular surface structures are revealed. Meanwhile, the accuracy of the numerical model is tested. In general, the numerical simulation results match the experimental observation well. However, there are several important discrepancies. For each discrepancy the possible causes are discussed, which benefits the further attempts of the modeling work on the lip seal running against surface textured shafts. The conclusions of this study themselves can be used as a guidance to the design of the surface textured shafts for the lip seal applications. Finally the limitation of the current theories and the modeling methods are discussed and reasonable improvements which can be done are proposed for the future work

Analysis of helical gear performance under elastohydrodynamic lubrication

In this thesis an elastohydrodynamic lubrication (EHL) solution method has been developed for helical Gears. Helical gears mesh with each other and develop contact areas under load that are approximately elliptical in shape. The contact ellipses have aspect ratios which are large and lubricant entrainment takes place in the rolling / sliding direction which is along the minor axis of the contact ellipse. The contact between helical gear teeth is therefore considered as a point contact EHL problem and the EHL analysis has been developed to include all aspects of the correct gear geometry. This includes the variation in radius of relative curvature at the contact over the meshing cycle, the introduction of tooth tip relief to prevent premature tooth engagement under load, and axial profile relief to prevent edge contact at the face boundaries of the teeth. The EHL solution is first obtained as a quasi-steady state analysis at different positions in the meshing cycle and then developed into a transient analysis for the whole meshing cycle. The software developed has been used to assess the effects of geometrical modifications such as tip relief and axial crowning on the EHL performance of a gear, and different forms of these profile modifications are studied. The analysis shows that the transient squeeze film effect becomes significant when the contact reaches the tip relief zone. Thinning of the film thickness occurs in this region and is associated with high values of pressure which depend on the form of tip relief considered. A transient EHL analysis for helical gears having faceted tooth surfaces has also been developed. Such surface features arise from the manufacturing process and can have a significant effect on the predicted transient EHL behaviour. The EHL results have been found to depend significantly on the facet spacing and thus on the manufacturing process. The important effect of surface roughness is also considered by developing a three dimensional line contact model to include real surface roughness information by considering a finite length of the nominal contact in the transverse direction of the tooth. This model is based on the use of the fast Fourier transform method to provide the repetition of the solution space along the nominal contact line between the helical teeth with the inclusion of cyclic boundary conditions at the transverse boundaries of the solution space. In helical gears the lay of tooth roughness (direction of finishing) is generally inclined to the direction in which rolling (entrainment) and sliding take place, and this is found to have a significant effect on both film thickness and pressure distribution

Measurement of deformation in rolling and sliding contacts

In this work, mechanisms behind micro-scale changes on the surfaces in\ud rolling and sliding contacts are studied both experimentally and\ud numerically. For the experimental study a wear and deformation\ud measurement system is designed and produced. This system is composed of\ud an interference microscope, a controllable rotating table and a friction\ud device. The system allows measurement of the micro and nano-scale local\ud changes on the surface micro-geometry during a wear or deformation\ud experiment