Rolling-sliding Contact Research Articles

Effect of oil starvation on the surface dimple in elastohydrodynamic lubrication contact under opposite sliding conditions

Optical interferometry experiments were performed to investigate the dimple phenomena in starved elastohydrodynamic lubrication of point contacts under opposite sliding condition by using a high viscosity polybutene (PB) lubricant. The effects of the bilateral starvation, left starvation, and right starvation on the variation of the oil film under a high slide-to-roll ratio were investigated. It was found that oil starvation could generate a starved constriction, which would make the shape of the oil film resemble the one in pure-rolling or rolling-sliding contact. However, the film thickness at the starved constriction is much lower. In addition, the influence of the applied load on the variation of the surface dimple in fully flooded and starved conditions was explored. It was found that the effect of oil starvation depended on the way the net mass flow through the contact zone. When the net mass flow was from left to right, a major constriction was formed on the right side, and vice versa. When symmetrically increasing the degree of oil starvation, the classical big dimple shrunk and became centralized before eventually disappearing and the contact approximated the nominally flat state of a heavily starved or almost dry contact.

Friction prediction of rolling-sliding contact in mixed EHL

Abstract A new numerical model for the friction prediction of the mixed elastohydrodynamic lubrication (EHL) is presented. The overall friction coefficient was calculated by integrating the uniform shear stress contributed by the fluid film and asperity interactions. Numerical results demonstrate that: (1) a large load often induces a large friction coefficient, (2) an increase in the surface roughness increases the friction coefficient both in mixed EHL and full film EHL, and (3) the rolling velocity decreases the friction coefficient in mixed EHL due to the increases in hydrodynamic effect and lubricant film thickness. In addition, a stribeck curve is presented to reveal the transition of friction and lubrication conditions from boundary lubrication to mixed EHL and full film EHL. The current approach has proven to be able to predict the friction coefficient over a wide range of loading conditions.

Design of dimpled engineering surfaces for improving lubrication performance in rolling-sliding contacts

Functional performance depends largely on the surface characteristics of critical components. This paper describes the design of dimpled engineering surfaces and the fabrication of micro-patterns on steel samples. The designed engineering surfaces are analysed and evaluated in terms of their mixed EHL film thicknesses and pressure distributions in the elliptical Hertzian contact zone.

Design of dimpled engineering surfaces for improving lubrication performance in rolling-sliding contacts

Design of dimpled engineering surfaces for improving lubrication performance in rolling-sliding contacts

Numerical Investigation of Ratchetting Behaviour in Rail Steel under Cyclic Rolling-Sliding Contact

This paper presents a numerical investigation to describe ratchetting behaviour in rail steel under cyclic rolling-sliding contact state. The study numerically quantifies cyclic plastic strain accumulation in rail material. This study combines the Hertzian contact pressure, longitudinal tangential traction based on Carter's theory and heat flux distribution across the contact patch to simulate the wheel-rail contact problem. In the numerical procedure, the effective loading is translated on the rail surface for twelve wheel passes. Temperature dependent cyclic plasticity material model of Chaboche featuring non-linear kinematic hardening is employed that simulates the ratchetting behaviour of the rail material. Results are extracted in terms of plastic strain and stress-strain response. These results are able to augment the understanding of failure mechanisms in rail material that arise from friction based thermo-mechanical load at the dynamic wheel-rail contact interface. The knowledge evolved can provide useful information to the development and application of rail steels and the development of effective rail maintenance strategies in order to mitigate rail material degradation.

A coupled finite-volume CFD solver for two-dimensional elasto-hydrodynamic lubrication problems with particular application to rolling element bearings

This paper describes a new computational fluid dynamics methodology for modelling elastohydrodynamic contacts. A finite-volume technique is implemented in the ‘OpenFOAM’ package to solve the Navier-Stokes equations and resolve all gradients in a lubricated rolling-sliding contact. The method fully accounts for fluid-solid interactions and is stable over a wide range of contact conditions, including pressures representative of practical rolling bearing and gear applications. The elastic deformation of the solid, fluid cavitation and compressibility, as well as thermal effects are accounted for. Results are presented for rolling-sliding line contacts of an elastic cylinder on a rigid flat to validate the model predictions, illustrate its capabilities, and identify some example conditions under which the traditional Reynolds-based predictions deviate from the full CFD solution.

Tribochemical study of micropitting in tribocorrosive lubricated contacts: The influence of water and relative humidity

Water ingress into the lubricant as a contaminant affects performance leading to an alteration in wear, corrosion and fatigue behaviour of the tribological components especially in the rolling element bearings. The current study addresses the tribochemical phenomena involved in micropitting in tribocorrosion systems where different levels of dissolved-water are present in a model lubricant. In this study the effect of different temperatures, water concentrations and relative humidities have been investigated on micropitting under rolling-sliding contacts. The influence of free and dissolved water on tribocorrosive micropitting is clarified. The tribochemical change of the reaction films is studied using X-ray Photoelectron Spectroscopy (XPS) which confirmed that the (poly)phosphate chain length and tribofilm thickness are reduced with increased dissolved-water level.

Investigation of the effect of a diamine-based friction modifier on micropitting and the properties of tribofilms in rolling-sliding contacts

The effect of N-tallow-1,3-diaminopropane (TDP) on friction, rolling wear and micropitting has been investigated with the ultimate objective of developing lubricants with no or minimal environmental impact. A mini traction machine (MTM-SLIM) has been utilised in order to generate tribofilms and observe the effect of TDP on anti-wear tribofilm formation and friction. Micropitting was induced on the surface of specimens using a micropitting rig (MPR). The x-ray photoelectron spectroscopy (XPS) surface analytical technique has been employed to investigate the effect of TDP on the chemical composition of the tribofilm while atomic force microscopy (AFM) was used to generate high resolution topographical images of the tribofilms formed on the MTM discs. Experimental and analytical results showed that TDP delays the zinc dialkyldithiophosphate (ZDDP) anti-wear tribofilm formation. TDP in combination with ZDDP induces a thinner and smoother anti-wear tribofilm with a modified chemical structure composed of mixed Fe/Zn (poly)phosphates. The sulphide contribution to the tribofilm and oxygen-to-phosphorous atomic concentration ratio are greater in the bulk of the tribofilm derived from a combination of TDP and ZDDP compared to a tribofilm derived from ZDDP alone. Surface analyses showed that utilising TDP effectively mitigates micropitting wear in the test conditions used in this study. Reduction of micropitting, relevant to rolling bearing applications, can be attributed to the improved running-in procedure, reduced friction, formation of a smoother tribofilm and modification of the tribofilm composition induced by TDP.

Modeling of elastic finite-length space rolling-sliding contact problem

This paper presents a semi-analytical method (SAM) to solve sliding finite-length space contact problem with consideration of the effect of free end surface. The equation set based on the overlapping concept is analytically solved with tangential load considered and the explicit expressions for equivalent mirrored loads are obtained. Conjugate gradient method (CGM) is adopted to solve the pressure distribution and Fast Fourier transform (FFT) is used to speed up stress related calculation. The model is verified by FEM and shows a high conformity. Different line contact situations are discussed to explore the effect of free end surfaces.

Microstructural Evolution of a Hypoeutectoid Pearlite Steel under Rolling-sliding Contact Loading

To study the microstructural evolution of pearlite steel subjected to pure rolling and rolling-sliding contact loading, a hypoeutectoid pearlite steel with composition and microstructure similar to BS11 was designed and twin-disc tests of this pearlite steel were performed to simulate the wheel/rail system. After a series of twin-disc tests, optical microscope (OM) observation, scanning electron microscope (SEM) observation, X-ray diffraction (XRD), and micro-hardness tests were conducted to characterize the microstructure. Under the pure rolling contact condition, a large amount of reticular cracks emerged within 60 μm below the contact surface of the samples after 120000 revolutions. The largest deformation was approximately 200 μm below the contact surface. Under the rolling-sliding contact condition, the nodularization of pearlite within 100 μm below the contact surface was obvious. The microstructure and stress-strain distribution of the area within 2 mm below the contact surface were investigated. The distribution of micro-hardness under the contact surface varied with contact conditions. Finite element method (FEM) was used to simulate the stress-strain distribution. The results of SEM, FEM, and micro-hardness tests indicated that under the pure rolling contact condition, the maximum plastic strain was approximately 200–400 μm below the contact surface. Conversely, under the rolling-sliding contact condition, the maximum plastic strain emerged on the contact surface. Under the pure rolling contact condition, the distribution of micro-hardness was almost identical to that of the equivalent plastic strain. Under the rolling-sliding contact condition, the distribution of micro-hardness was affected by the equivalent plastic strain and tangential stress.

Traction formula for rolling-sliding contacts in consideration of roughness under low slide to roll ratios

Based on the numerical results of a wide range of parameters in terms of the linear complementarity framework, an easy-to-use formula is presented by using the regression analyses to estimate the traction coefficient for rolling-sliding contacts under low Slide-to-Roll Ratio (SRR) domain for dry condition. The parameters include geometry, surface roughness, SRR and friction coefficient and contact radius. The formula's predictions are in good agreement with measurements using a ball on disc test rig. The results indicate that for steel-steel contact, the effects of third body layer and plasticity on the traction can almost cancel each other out. In addition, by revising the expression proposed by Masjedi and Khonsari, the formula is extended to estimate the cases under mixed lubrication.

Effects of material heterogeneity on surface fatigue for rough lubricated rolling–sliding contacts

In view of increasingly severe operating conditions and the use of composite (strongly) heterogeneous materials, detailed modelling and optimisation methods are needed to predict the effects of subsurface material topology, either by design or resulting from inclusions and material anisotropy, on rolling-sliding contact fatigue life. In this paper, a method is proposed showing that such predictions can at present be obtained for realistic configurations on small-scale computers by integrating efficient numerical solution of the 3D displacement and stress field in the (heterogeneous) material with fast rough surface-lubricated contact models. Contact pressures, subsurface stresses and surface fatigue effects are shown for cases of bonded individual or multiple (clusters) of statistically distributed inhomogeneities close to the surface in realistic actual rolling–sliding rough contact geometries. The model contributes to the development of optimised failure criteria for composite/heterogeneous materials close to the surface.

Mechanical model of coupling rolling and sliding friction in real-time non-clearance precision ball transmission

In this paper, a new static mechanical model is established to study the rolling-sliding contact in the real-time non-clearance precision ball drive. Meshing forces at the main and the secondary transmission force points in two types of mechanical models are deduced shown in moment equilibrium equations and energy balance equations. Based on coupling coefficient determined by the Carter narrow theory, a new mechanical model of coupling rolling and sliding friction is established. The meshing force simulated by the finite element method has good matching with theoretical calculation with error less than 5.6%, which verifies the feasibility of the coupled model. Outcome of the research could be a theoretical support in improving the transmission accuracy of the drive.

Wear Behavior of Martensitic Stainless Steel in Rolling-Sliding Contact for Planetary Roller Screw Mechanism: Study of the WC/C Solution

Wear Behavior of Martensitic Stainless Steel in Rolling-Sliding Contact for Planetary Roller Screw Mechanism: Study of the WC/C Solution

Fluid Pressurization and Entrapment Effects on the SIFs of Cracks produced under lubricated Rolling-Sliding Contact Fatigue

Pitting is one of the causes of failure for mechanical components subjected to rolling contact fatigue. In the present article, a FE model is described in which a 2D half-space with an edge crack is affected by a travelling contact load produced by a cylindrical body. The contact load is not approximated as usual by an analytical pressure distribution but the actual mating body is modelled. The presence of lubricant between the mating bodies and inside the crack is taken into account and its effect on the crack is modelled via hydrostatic elements. The lubricant is assumed to be entrapped into the crack by the external body when the latter covers the crack mouth, that is, the crack is sealed by the contact area and not by the contact between the crack faces (fluid entrapment mechanism). The pressure of the fluid is calculated via an iterative procedure by assuming that its volume stays constant inside the crack. Comparisons between this model and the alternative fluid pressurization mechanism have been made. The outcomes suggest that the fluid pressures inside the crack produced by the fluid entrapment mechanism tend to those of the fluid pressurization mechanisms as the crack becomes short.

歯車用高強度鋼材の接触・曲げ複合疲労試験

Many high strength steels for gear materials have been launched in these days. Conventional material tests; e.g., pulsator test for tooth root bending strength and twin-disc tests for rolling-sliding contact strength, might not accurately evaluate their strength in gear teeth. It is because a failure mode which frequently appears on gear teeth made of a high strength steel is not so simple as a breakage and pitting, but is a combination of these two modes. This failure mode could be simulated with contact-bending-fatigue (CBF) tests in which a test piece is subject to a bending stress similarly in the rotary-bending-fatigue tests and a contact pressure similarly in twin-disc tests. The present paper describes the concept of the CBF test and its possibilities for evaluating high strength gear materials.

The concept of the average stress in the fracture process zone for the search of the crack path

The concept of the average stress has been employed to propose the maximum average tangential stress (MATS) criterion for predicting the direction of fracture angle. This criterion states that a crack grows when the maximum average tangential stress in the fracture process zone ahead of the crack tip reaches its critical value and the crack growth direction coincides with the direction of the maximum average tangential stress along a constant radius around the crack tip. The tangential stress is described by the singular and nonsingular (T-stress) terms in the Williams series solution. To demonstrate the validity of the proposed MATS criterion, this criterion is directly applied to experiments reported in the literature for the mixed mode I/II crack growth behavior of Guiting limestone. The predicted directions of fracture angle are consistent with the experimental data. The concept of the average stress has been also employed to predict the surface crack path under rolling-sliding contact loading. The proposed model considers the size and orientation of the initial crack, normal and tangential loading due to rolling–sliding contact as well as the influence of fluid trapped inside the crack by a hydraulic pressure mechanism. The MATS criterion is directly applied to equivalent contact model for surface crack growth on a gear tooth flank.

Minimizing Frictional Losses in Crankshaft Bearings of Automobile Powertrain by Diamond-like Carbon Coatings under Elasto-hydrodynamic Lubrication

Abstract An efficient powertrain technology contributes to sustainable lowering of greenhouse gas emissions in terms of reducing mechanical losses and saving fossil fuels. Minimization of frictional losses of highly loaded components in automobile powertrain as crankshaft plain bearings and gears offers massive potential for target achievement. Application-related investigations of diamond-like carbon (DLC) coatings ZrCg (a:C-H/ZrCg) and nanocomposite (nc)-ZrC (a‑C:H/ZrC) in lubricated rolling-sliding contacts in twin-disc and gear efficiency test-rig revealed great potential of ZrCg for efficiency improvement in automobile gearboxes by reducing mechanical losses up to 39 % under elasto-hydrodynamic lubrication (EHL) compared to uncoated gears. With regard to plain bearings, ZrCg and nc-ZrC coated modified prototype bearings exhibited a friction advantage of up to 36 % in EHL compared to standard bearings in an engine test bench. This yet largely unknown favorable effect of DLC coatings under rolling-sliding conditions in EHL opening doors to new possibilities in tribology and efficiency improvement was attributed to the thermophysical properties of DLC coatings and was confirmed by simulations of rolling-sliding contacts.

Wear of Carbon Steel (0.65%C) in Rolling-Sliding Contact with Creep Ratio

In rolling-sliding contact, wear will occur when the accumulated plastic shear strain of the material at the surface exceeds its critical shear strain for failure. During rolling-sliding contact, the difference in relative velocities of two contacting components can cause slip in the contact (known as creep). The higher creep ratio may increase the severity of wear. In this work, the wear rate of the material and the behaviour of material just below the contact surface in rolling-sliding contact with various creep ratios were investigated. The carbon steel (about 0.65% C) was chosen as the test material and wear test was conducted using disc-to-disc contact testing machine with the maximum contact pressure of 1000 MPa and with various creep ratio of 1%, 5% and 7%. The results show the higher creep ratio causes the material to accumulate critical shear strain more quickly, resulting in the increase of wear (i.e., from about 0.0047μm/cycle for creep ratio of 1% up to about 0.0077μm/cycle for creep ratio of 7%). .

Interaction of DLC and B4C coatings with fully formulated oils in boundary lubrication conditions

Diamond-like carbon (DLC) and Boron Carbide (B4C) coatings are promising candidates for sliding–rolling contact parts, especially gears to increase scuffing load capacity and prolong lifetime. Generally, commercially available fully formulated oils are compatible with ferrous surfaces, but normally have not been optimised for coatings. In this study, the tribological properties of tungsten carbide containing DLC (WC-DLC), hydrogenated DLC (H-DLC) and B4C coatings against steel ball with two fully formulated oils in boundary lubrication conditions have been evaluated. They were compared with that of a Steel/Steel tribocouple. Interactions between lubricants and sliding surfaces were also investigated using surface characterisation techniques such as XPS and Raman Spectroscopy. It was found that Steel/H-DLC system provided the lowest friction coefficient for both oils.