Slide-to-roll Ratio Research Articles

Effect of slide to roll ratio on the micropitting behaviour in rolling-sliding contacts lubricated with ZDDP-containing lubricants

Abstract An automated micropitting test rig was applied to study the effect of slide to roll ratio (SRR) on micropitting behaviour in rolling-sliding contacts lubricated with Zinc Dialkyl Dithiophosphate (ZDDP) containing lubricants. The tested specimen surface was examined under an optical microscope and Scanning Electron Microscope(SEM) to study the micropitting behavior of the surface, followed by applying X-ray Photoelectron Spectroscopy(XPS) to study the related tribochemistry behavior. The results show that the increase of SRR can reduce the number of micropits on the worn surface while wear increased and friction did not change much. XPS results indicate that the increase of SRR resulted in the increase of oxide concentration while the decrease of film thickness on the worn surface.

Effect of Thermal Properties of a Coated Elastohydrodynamic Lubrication Line Contact Under Various Slide-to-Roll Ratios

A numerical thermal elastohydrodynamic lubrication (EHL) model is developed for coated line contacts by considering both the mechanical properties and the thermal properties of the coating and the substrate. The temperature fields within the oil film and within the solids are solved by deriving the energy equations for the solids and the oil film. Heat continuity conditions are satisfied at the interfaces between the solids and the oil film, and the coating/substrate interfaces. Effects of the slide-to-roll ratio (SR), the thermal conductivities of the coating bodies, and the oil film on temperature fields are studied.

Two phosphonium cation-based ionic liquids used as lubricant additive: Part I: Film thickness and friction characteristics

The lubricant film thickness and friction properties of a mineral base oil and its mixtures with two phosphonium cation-based ionic liquids: [P66614][(iC8)2PO2] and [P66614][BEHP] at 0.5 and 1wt% concentrations were studied under different slide-to-roll ratios (SRR) and temperatures in an EHD2 ball-on-disc test rig. All lubricant samples showed similar lubricant film thickness in full film lubrication and behaved similarly in friction tests with polished discs at SRR of 5%, but the mixtures outperformed the neat base oil at SRR of 50%. The mixtures were also better in tests with rough discs. Tribological improvements with mixtures were achieved under mixed and boundary lubrication regimes. Real application such as rolling bearings will be presented in the second part of this work.

Particle Entrapment in Rolling Element Bearings: The Effect of Ellipticity, Nature of Materials, and Sliding

ABSTRACTThis study on particle entrapment within elastohydrodynamic contacts reports experimental tests mimicking rolling element bearing contacts using a twin-disc machine with a contaminated lubricant as well as numerical simulations. The ball bearing contacts might be nonideally circular, composed of a hybrid couple of contact materials, and operated under rolling–sliding conditions. This study focused on the relative importance of each of these configurations to the mechanisms responsible for particle entrapment. It was found experimentally and confirmed numerically that the contact ellipticity is of primary importance to entrapment probability. Tests mixing two-by-two pure rolling and non-zero slide-to-roll ratios (SRRs) and different materials and contact configurations revealed combined mechanisms leading to entrapment and surface damage.

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.

The Influence of Sliding and Contact Severity on the Generation of White Etching Cracks

White etching cracks (WECs) have been identified as the dominant mechanism of premature failure for bearings within wind turbine gearboxes. Though WECs have been observed in the field for over a decade, the exact mechanisms which lead to this failure are still debated, and benchtop replication has proven difficult. In previously published work, WECs have been replicated only through the use of component level test rigs, where complete bearings are tested. In these tests, the factors that are thought to drive the formation of WECs, such as slide-to-roll ratio (SRR) and lubricant film thickness, cannot not be easily altered or controlled. In this paper, WECs have been replicated on a three ring on roller, benchtop test rig, which allowed for a direct investigation into the influence that SRR magnitude, sliding direction, and the lubricant film thickness have on surface failures and WEC generation. It was determined that WECs were formed in samples that experienced −30 % SRR at various lubrication conditions; however, at lower levels of negative SRR and positive SRR up to 30 %, no white etching cracks were observed.

On the friction behaviour of polymer greases

Non-additized batches of lubricating greases were tribologically characterized through traction coefficient and Stribeck curves at different operating conditions on a ball-on-disc test rig, ensuring that the contact was fully flooded. The tests were performed at constant load, but different operating temperatures while varying the slide-to roll ratio (SRR) or the rolling entrainment speed.The results were correlated to the greases׳ formulation in terms of base oil viscosity, thickener and/or elastomer content. A relationship between the coefficient of friction (COF) of greases formulated with different thickener content was found and the thickener influence on the COF was addressed.

Effect of the Slide-to-Roll Ratio and the Contact Kinematics on the Elastohydrodynamic Friction in Diamond-Like-Carbon Contacts with Different Wetting Behaviours

In this paper, we show how the slide-to-roll ratio (SRR), the contact kinematics and the surface energy all have important effects on the elastohydrodynamic friction. As reported previously, diamond-like-carbon (DLC) contacts of the type DLC/DLC provide the lowest coefficient of friction, in particular those DLC materials with the lowest surface energies (three different DLC coatings were used in this study). A friction reduction of up to 48 %, compared to a steel/steel contact, was obtained. A surprising new finding from this investigation is that the friction in DLC contacts is significantly reduced for high SRRs, i.e. a difference of up to 27 % was measured in the DLC/DLC contacts when the SRR increased from 0.5 (rolling prevails) to 1.8 (sliding prevails). However, even more surprising is the effect of the SRR on the friction in mixed steel/DLC contacts, which is related to the contact design and its kinematics. We found that if the DLC is coated on the slower surface, a high SRR will not reduce the friction to any significant extent, typically about 5 %, and at most 13 %, compared to steel/steel contacts. However, a significant friction reduction is observed when applying a low-surface-energy DLC coating to the faster surface—on average about 20 % and as high as 33 %. In this case (a high SRR and DLC on the faster surface), mixed steel/DLC surfaces can experience a very similar friction as that seen for DLC/DLC contacts. Qualitatively, the same behaviour was found for both positive and negative SRR values.

Tribology of particle suspensions in rolling-sliding soft contacts

We investigate the lubrication of microsphere suspensions between compliant substrates, and probe the influence of matrix viscosity, particle phase volume, surface roughness and wetting, and slide-to-roll ratio (SRR). In general, the suspensions behave as a continuum in the elastohydrodynamic regime provided the film thickness, which is predicted from the product of speed and viscosity, is greater than the particle diameter. Below this, the frictional response is characteristic of the mixed and boundary regimes. In the boundary regime, friction is independent of phase volume above 5% and it is governed by the rolling friction associated with particles being entrained into the contact that is independent of SRR, which is made possible by substrate deformation. This study provides a benchmark for soft-tribology and biotribology studies involving more complex particle suspensions and particle-containing soft materials.

Characteristic times in transient thermal elastohydrodynamic line contacts

For practical applications, there are several points of interest in the numerical simulation of transient Thermal Elastohydrodynamic (TEHD) lubrication. Among others, is the possibility to investigate the impact of transient effects depending on the dynamic characteristics of the contact. In this regard, the paper presents an original study on the characteristic times involved in a generic line contact lubricated with a non-Newtonian fluid. The paper highlights the dependency of the dominant heat transfer mode on the Slide-to-Roll Ratio (SRR), leading to the formulation of new thermal characteristic times. Based on results from time-dependent computations, it also provides a criterion linking the onset of transient effects to the dominant thermal characteristic time which has been validated on various operating conditions.

Minimum quantity lubrication of M50/M50 and M50/Si3N4 tribological interfaces

Abstract Hybrid rolling element bearings are being considered for use in flight critical aerospace applications. The following work was undertaken to lay the foundation for future work that would permit the recommendation of appropriate quantities of lubrication for these advanced bearing systems. This was accomplished by focusing on the tribological performance of M50/M50 (or AISI M-50) and M50/Si3N4 interfaces with minimum quantities of lubrication at two levels of speed and slide-to-roll ratio (SRR). Ball-on-ring tribological testing was conducted with stepped reductions of lubricant pump rate into the tribological contacts at initial lambda values of approximately 1.7 and 2.5, as calculated from the Hamrock and Dowson film thickness equation and surface finish measurements. Experiments were conducted with 5% and 45% slide-to-roll ratio (or SRR). Starting from these high initial lambda levels, the lubricant flow rate into the contact was reduced until severe damage occurred. Tests were then suspended due to excessive traction (traction coefficient greater than 0.2). The results of this work are in agreement with the starved lubrication literature and the bearing test literature. They provide some solid insight into the minimum quantity of lubrication recommended for M50/M50 and M50/Si3N4 interfaces. In all of the tests conducted there was a clear trend that higher speed and increased sliding both require an increased supply of lubricant to the contact. The M50/M50 interface needed an order of magnitude higher lubricant pump rate into the contact for adequate lubrication as compared to the M50/Si3N4 interface in the initial lambda ∼ 1.7 experiments. At lambda ∼ 2.5 with 5% SRR, the M50/M50 interface needed three times the lubricant pump rate that the M50/Si3N4 interface needed. However, both M50/M50 and M50/Si3N4 needed a similar amount of lubricant at lambda ∼ 2.5 with 45% SRR, in which the interfaces were severely damaged due to the extreme test conditions. These tribological test results are interpreted with an analysis of the wear morphology and surface chemistry of the test articles.

Elastohydrodynamic lubrication friction mapping – the influence of lubricant, roughness, speed, and slide-to-roll ratio

A friction test is conducted in a Wedeven Associates Machine ball-on-disc test rig. The output from the test, friction coefficient versus entrainment speed and slide-to-roll ratio (SRR), is presented as a three-dimensional friction map. A number of parameters are varied while studying the friction coefficient; surface roughness, base oil viscosity, base oil type, and extreme pressure (EP) additive package. Entrainment speed, SRR, and oil temperature are also varied. The results show that the mapping is efficient in showing the different types of friction that may occur in an elasto-hydrodynamic lubrication contact. The results also show that the friction behaviour can be strongly influenced by changing surface roughness as well as base oil viscosity, base oil type, EP additive content, and operating temperature.

Computational fluid dynamics and full elasticity model for sliding line thermal elastohydrodynamic contacts

Classically, the EHD problem is solved using the Reynolds assumptions to model the fluid behaviour, and the Boussinesq elastic deformation equation to model the solid response, both being coupled with the load balance equation. The development of an alternative approach is presented here in order to solve at once the Navier–Stokes equations (mass conservation and momentum equilibrium), the full elasticity and energy equations for the line EHD problem in a fluid–structure interaction approach. The Finite Element Method is used to solve the mathematical formulation in a fully coupled way, inspired from Habchi et al. (2008) [1]. After linearisation with the Newton procedure, all the physical quantities (pressure, velocity field, deformations and temperature) are solved together in a unique system. An important benefit of this approach is the possibility to implement in a simple manner the non-Newtonian and thermal effects; in fact all the quantities can vary through the film thickness. The extension to non-Newtonian rheology and the pressure and temperature dependencies for the viscosity and density are taken into account in a direct way to allow an acceptable prediction of the friction coefficient. Gradients across the film thickness and temperature fields in both the fluid and the two solids are naturally computed and analysed. As a case study, we focus first on the pure sliding cylinder-on-plane contact. It is shown that thermal effects due to friction in the central zone of the contact play a role in heating the lubricant at the inlet zone, via heat conduction in the solids. By increasing the Slide-to-Roll Ratio (SRR), the occurrence of dimples and the subsequent effects in different parts of the contact under zero entrainment velocity conditions are then studied.

Effect of lubricant additives in rolling contact fatigue

Lubricant formulations for manual gear box applications are optimized for gear contacts although rolling element bearings are lubricated with the same fully formulated oil. This may cause fatigue-related damage to these bearings. Several explanations can be considered, but this work focuses on the effect of additives contained in the lubricant. Rolling contact fatigue tests were performed on a twin-disc machine. Artificial dents generated by a Rockwell penetrator were made on the faster surface in order to accelerate the fatigue phenomena. Then, pure rolling and 6.7 per cent slip (slide-to-roll ratio (SRR)) tests were performed with different lubricants (pure base oil, fully formulated oil, and base oil with detergent and anti-foam). Fatigue life results and spalling morphologies are compared. For the sample obtained with the fully formulated oil and 6.7 per cent of SRR, crack analysis was performed. Using focus-ion-beam technique, a spalled sample was milled to reveal a cross-section of a crack. Secondary electron microscopy (SEM) images were taken and energy-dispersive X-ray spectrometry (EDX) analyses along the crack were performed. Additive elements are detected up to the crack tip. Auger electron spectroscopy depth profiling was also performed in the tribofilm generated on the disc surface. The role of additives in rolling contact fatigue is discussed in the light of these results.

Computation of fatigue strength in case-hardened cylindrical components using the local fatigue strength concept: [Berechnung der Dauerschwingfestigkeit Einsatzgeharteter Zylindrischer Korper mit dem Konzept der Lokalen Dauerfestigkeit]. Kuttner, T. and Zieger, H. Harterei-Technische Mitteilungen (Nov.–Dec. 1992) 47 (6), 367–375 (in German)

Rolling contact fatigue (RCF) regularly causes mechanical components to fail under cyclic loads. Popular surface treatment processes such as case hardening (carburization), nitriding and tempered martensitic reduce the effect of RCF damage. This paper presents multi-stage S-N curve for steels with different surface treatments, under point and line contact conditions using RCF tribometers for the experiments and finite element (FEM) simulation to simulate localised stresses in the contacts. Additionally, S-N tests with 0, 5 and 11% slide to roll ratio (SRR) are presented. This work demonstrates the importance of testing key points at RCF S-N curves to optimize the hardened layers.

Compressible elastohydrodynamic lubrication of rolling and sliding contacts with a power law fluid

This paper presents a theoretical solution for compressible elastohydrodynamic lubrication (EHL) using power law fluids. The performance characteristics of EHL line contacts are investigated for the power law index n in the range from 0.8 to 1.2 and the slide-toroll ratio in the range from 0.1 to 1.9. Numerical results show that, with increase in the power law index n, the pressure spike moves toward the centre of contact and the location of the cavitation moves away from the centre of contact for compressible fluids. As the slide-to-roll ratio increases, the minimum film thickness becomes thin when n < 1 and conversely becomes thick when n > 1. The sliding friction coefficient increases as the slide-to-roll ratio increases for all power law indices n.