A non-averaging method of determining the rheological properties of traction fluids

Traction machines have been frequently used to study the rheological responses of lubricants in elastohydrodynamic lubrication (EHL) contacts. Fundamental properties are inferred from EHL traction measurements based on the average pressures and temperatures in the contact. This average approach leads to uncertainty in the accuracy of the results due to the highly nonlinear resonse of the fluid such as viscosity to both pressure and temperature. A non-averaging method is developed in this paper to study the elastic and plastic properties of traction fluids operating in EHL contacts at small slide-to-roll ratios. A precision line-contact traction rig is used to measure the EHL traction at a given oil temperature and Hertz pressure. By choosing a sensible pressure-property expression, the parameters of the expression can be determined through the initial slope and peak traction coefficient of the traction measurements. The elastic shear modulus and the limiting shear stress of the lubricant corresponding to a single pressure can then be calculated for a range of pressures and temperatures of practical interest. Two high-traction fluids are studied, and their elastic moduli and limiting shear stresses presented.     

Compression Heating and Cooling in Elastohydrodynamic Contacts

In this study, the infrared temperature mapping technique, originally developed by Sanborn and Winer (Trans ASME J Tribol 93:262–271, 1971) and extended by Spikes et al. (Tribol Lett 17(3):593–605, 2004), has been made more sensitive and used to study the temperature rise of elastohydrodynamic contacts in pure rolling. Under such conditions lubricant shear heating within the contact is considered negligible and this allows temperature changes due to lubricant compression to be investigated. Pure rolling surface temperature distributions have been obtained for contacts lubricated with a range of lubricants, included a group I, and group II mineral oil, a polyalphaolefin (group IV), the traction fluid Santotrac 50 and 5P4E, a five-ring polyphenyl-ether. Resulting maps show the temperature rise in the contact increases in the inlet due to compression heating and then decreases and in most cases becomes negative in the exit region due to the effect of decompression. Temperature changes increase with entrainment speed but in the current tests are always very small, and less than 1 °C. Contact temperature rises from compression were compared to those from sliding contacts (where a slide-roll ratio of 0.5 was applied). Here the contribution to the contact temperature from compression is shown to decrease dramatically with entrainment speed. The lubricant 5P4E is found to behave differently from other lubricants tested in that it showed a peak in temperature at the outlet. This effect becomes more pronounced with increasing speed, and has tentatively been attributed to a phase change in the exit region. Using moving heat source theory, the measured temperature distributions have been converted to maps showing rate of heat input into each surface and the latter compared with theory. Qualitative agreement between theory and experiment is found, and a more accurate theoretical comparison is the subject of ongoing study.     

An efficient numerical analysis of starved thermohydrodynamically lubricated rolling line contacts

Effect of starvation in thermohydrodynamically lubricated high rolling speed line contacts has been investigated numerically by using an efficient numerical method in which temperature variations across the lubricant film is approximated by the second-order of Legendre polynomial. Mechanism of starvation at the contact has been set by creating gradual reduction in the length of the computational domain from the inlet side. In the solution, the lubricant has been assumed to be a Newtonian fluid. Minimum film thickness and rolling traction coefficient under fully flooded and starved conditions have been computed in this work. The rolling traction coefficient, minimum film thickness, and maximum mid film temperature rise in the starved line contact are found to be lesser than the fully flooded contact condition.     

Experimental investigation of temperature rise in elliptical EHL contacts

A toroidal traction drive continuously variable transmission (CVT) transmits power by the shearing action of lubricant film under heavy loads at contact points on the CVT rollers. In other words, rolling and sliding motions produce traction force. Furthermore, due to the geometry of the toroidal CVT, spin motion is produced at the contact points. These contact points, which are elliptical in shape, are where shear stress of the lubricant generates frictional heat. Temperature rise at the contact point has never been measured under the conditions of high rolling speed (velocity) and minute amounts of sliding (slippage), nor has the influence of spin motion on temperature rise been examined thus far. The authors et al. measured temperature distribution at contact points under conditions of high rolling speed and minute amounts of sliding, such as what is found in a toroidal CVT, using an FZG twin-disk test machine and thin-film sensors. The influence of spin motion on temperature rise was also experimentally investigated.     

Thermal analysis of an Eyring fluid in elastohydrodynamic traction

Recent research into elastohydrodynamic (EHD) traction has shown that under high pressure and isothermal conditions the flow properties of typical lubricants follow the Eyring relation between shear stress τ and shear rate /.γ: $\text{η/.γ = τ}{}_0\text{sinh(}\text{τ}\text{τ}{}_0\text{)}$ where η is the Newtonian viscosity and τ₀ a representative stress. The maximum in the traction curve arises from shear heating, and Crook's thermal analysis for a Newtonian lubricant has been modified to apply to an Eyring lubricant.In an EHD contact the pressure and hence the viscosity η vary from inlet to outlet, but it is shown that under the conditions of maximum traction it is sufficiently accurate to use the average values of η and τ₀ associated with the average values of pressure and temperature through the length of the contact. A simple formula can then be derived for the maximum traction coefficient in terms of the properties of the fluid (viscosity, pressure and temperature indices, and the representative stress τ₀ and the operating conditions (contact pressure, speed and film thickness).     

Lubricant oils additivated with polymers in EHD contacts: Part 1. Rheological behaviour

This paper reports on the influence of a polymer additive on the traction behaviour of a mineral oil investigated using a two‐disc machine at different temperatures and contact pressures. A semi‐empirical approach was used for determining the effective lubricant rheological parameters ‐ the elastic shear modulus, the viscosity of the lubricant, and the limiting and Eyring stresses ‐ in elastohydrodynamic contacts. Using this approach, the effect of polymer concentration on the rheological parameters that appear in both the Johnson‐Tevaarwerk and Bair‐Winer models was quantified. The influence of operating conditions, such as pressure, oil temperature, and polymer concentration, on the traction coefficient, limiting shear stress (from the Bair‐Winer model), Eyring stress (from the Johnson‐Tevaarwerk model), shear modulus, and apparent viscosity was also investigated.     

Rolling contact fatigue of case-hardened chromium steel

The rolling contact fatigue of case-hardened steel surfaces in lubricated heavily loaded contact was studied. Three different case-hardening treatments were tested with a ratio of slide to roll of ?5%. Other ratios of slide to roll were used to determine the effect of tangential traction on the rolling contact fatigue endurance. When the actual contact width was measured after testing, the scatter of the fatigue results was reduced. The depth of the maximum plastic strain was determined by measuring the hardness before and after testing and was found to correspond to the occurrence of the lowest ratio of the critical shear stress to the amplitude of the load-induced orthogonal shear stress. The role of residual stresses in rolling contact fatigue is discussed. It was found that a more detailed knowledge of lubricant behaviour in heavily loaded contacts is needed to reveal the true distribution of tangential traction on the contact surface. This affects the angle of the plane and the value of the maximum amplitude of the shear stress beneath the contact zone.     

Study of Boundary Slippage Using Movement of a Post-Impact EHL Dimple Under Conditions of Pure Sliding and Zero Entrainment Velocity

A well-recognized phenomenon of typical traction tests of elastohydrodynamic lubrication (EHL) contacts is finite maximum traction at increasing speeds, which led to the postulation that the limiting shear stress of liquid lubricants, a high-pressure rheological property, existed. If slippage occurs at the oil–solid boundary, the limiting traction measured is not necessarily an intrinsic property of the lubricant but rather a function of interfacial properties between the bounding solid surface and the lubricant. A recent report presented experimental evidence of boundary slippage at EHL contacts using a simple methodology based on differences in the speed of oil entrapment and the apparent entrainment. The reported experiments were carried out under pure sliding conditions. The phenomenon may also be explained by internal slippage in the bulk fluid film because of the limiting shear stress of the lubricant. To clarify this, similar experiments were repeated under zero entrainment velocity (ZEV) conditions. Evidence of the highly pressurized lubricant at the center of the oil entrapment sliding against the solid bounding surface was obtained. The purpose of this article is to discuss whether the slippage is attributed to the limiting shear stress of the oil or the critical shear stress of the oil/solid interfaces, and how to differentiate the magnitudes of the critical shear stress of the two bounding surfaces in a conventional optical EHL test rig.     

Grease degradation in a bearing simulation device

In this study, a ball-on-disc traction (MTM) device has been used to degrade grease under controlled thermal and shear conditions over an extended period. The tests were run under both fully flooded and semi-starved conditions to simulate the lubricant supply levels found in bearings. A simple lithium hydroxystearate grease with and without an additive package was used. At the end of the test Infrared micro-reflection spectra were taken from the track and surrounding grease to determine lubricant film composition. These results were compared to IR reflection analysis of lubricant films in used bearings. The MTM results show that in the semi-starved tests the grease “runs-in” during the first few cycles as the friction coefficient drops giving a low, stable value comparable to the fully flooded condition. During initial overolling the grease is shear degraded releasing mobile lubricant, which replenishes the contact and reduces starvation. IR analysis of the lubricant film in and around the track has shown evidence of the local degradation, the formation of thickener-rich layers and new chemical species. The unadditised grease failed towards the end of the test due to incipient starvation, which was exacerbated by the formation of oxidised thickener deposits in the track. The IR spectrum from this film was very similar to that found in some bearing samples.     

Non-newtonian thermal analysis of an EHD contact lubricated with MIL-L-23699 oil

A thermal and non-Newtonian fluid model under elastohydrodynamic lubrication conditions is proposed, integrating some particularities, such as the separation between hydrodynamic and dissipative phenomena inside the contact. The concept of apparent viscosity is used to introduce the non-Newtonian behaviour of the lubricant and the thermal behaviour of the contact into the Reynolds equation, acting as a link element between the hydrodynamic and dissipative components of the EHD film, independently of the rheological and thermal models considered. The apparent viscosity enables the application of the rheological model better adapted to each lubricant, without appealing to special formulations of the EHD problem.The Newton–Raphson technique is used to obtain the lubricant film geometry and the pressure distribution inside the EHD contact. The shear stresses developed in the fluid film are evaluated assuming the non-linear Maxwell rheological model. The surfaces and lubricant temperature distributions are determined using the simplified Houpert's method, applied to the inlet contact zone, and the thermal method proposed by Tevaarwerk is applied in the high pressure contact zone.The non-Newtonian thermal EHD model is applied to the analysis of a contact lubricated with MIL-L-23699 oil. Significant results are obtained for the centre and minimum film thickness, for the inlet shear heating and film thickness reduction factor (φ_T), for the temperature rise of the lubricant and of the surfaces and for the friction coefficient inside the contact, considering wide ranges of the operating conditions (maximum Hertzian pressure, inlet oil temperature, rolling speed and slide-to-roll ratio).Finally, the numerical traction curves determined are compared with the corresponding experimental results, showing very good correlation.     

The Combined Role of Soot Aggregation and Surface Effect on the Friction of a Lubricated Contact

One of the considered research paths to reduce friction loss consists in optimizing the interactions between surfaces and lubricants. The latter may significantly change with the lubricant ageing. In this framework, the tribological behaviour of aged formulated lubricant is analysed for various low-speed reciprocating motions and with different nature of surfaces. This paper focuses on soot aggregate formation processes in a lubricated contact and on their correlation to friction. Although no aggregates have been observed in pure rolling conditions, pure sliding conditions may lead to the appearance of aggregates moving through the contact as a function of the nature of the surfaces. The analysis of their displacement within the contact is used to discuss their interactions with the surfaces. Moreover, we show that the velocity and the dwell time of the aggregate depend on the sliding speed. The morphology of these aggregates evolves over time, affecting friction behaviour. An additive law combining a contribution from the shear of the aggregates with another one due to the shear of a thin lubricant film surrounding the aggregate is then proposed to interpret friction origin and friction evolution with time of shear. The aggregate motion also varies with the nature of the surfaces: in particular, DLC–DLC couple reduces aggregation phenomena and maintains a low friction without apparent wear.     

Modeling Sliding Contact of Rough Surfaces with Molecularly Thin Lubricants

The sliding contact between two rough surfaces in the presence of a molecularly thin lubricant layer is investigated. Under very high shear rates, the lubricant is treated as a semi-solid layer with normal and lateral shear-dependent stiffness components obtained from experimental data. The adhesive force in the presence of lubricant is also adapted from the Sub-boundary lubrication model and improved to account for variation in surface energy with penetration into the lubricant layer. A model is then proposed, based on the Improved sub-boundary lubrication model, which accounts for lubricant contact and adhesion and its validity is discussed. The model is in good agreement with published experimental measurements of friction in the presence of molecularly thin lubricant layers and suggests that a molecularly thin lubricant bearing could be successfully used to reduce solid substrate damage at the interface.     

The Elastohydrodynamic Traction of Synthetic Base Oil Blends

The aim of this study was to investigate the elastohydrodynamic (EHD) properties of lubricant blends. Three base fluids of very similar viscosities, a polyalphaolefin, a diester and an alky lated aromatic, have been obtained and their EHD film thickness and traction behavior measured at a range of pressures. Blends of these fluids have been prepared and the influence of blending on film thickness and traction has been investigated. Traction measurements were conducted at film thicknesses between 100–200 nm and thermal analysis was incorporated to correct for in-contact shear heating. The blends showed a broadly linear relationship between the inlet pressure-viscosity coefficient and blend composition. Isothermal traction comparisons revealed that traction is not an additive property of lubricant blends.     

A Quantitative Solution for the Full Shear-Thinning EHL Point Contact Problem Including Traction

We present a realistic elastohydrodynamic lubrication (EHL) simulation in point contact using a Carreau-like model for the shear-thinning response and the Doolittle-Tait free-volume viscosity model for the piezoviscous response. The liquid lubricant modeled is a high-viscosity polyalphaolefin which has been shown by high-pressure viscometry to possess a relatively low threshold for shear-thinning as a single-component liquid lubricant. As a result, the measured EHL film thickness is about one-half of the Newtonian prediction. We derived and numerically solved the two-dimensional generalized Reynolds equation for the modified Carreau model based on Greenwood. In this simulation, viscosity was not treated as an adjustable parameter; the models used for the pressure and shear dependence of viscosity were obtained entirely from viscometer measurements. Truly remarkable agreement is found in the comparisons of simulation and experiment for traction coefficient and for film thickness in both pure rolling and sliding cases.     

Modeling Bearing and Shear Forces in Molecularly Thin Lubricants

Under the effects of high shear rate and confinement between solid surfaces, the behavior of a thin lubricant film deviates from that of the bulk, resulting in significant increases of lubricant viscosity and interfacial slip. A semi-empirical model accounting for the breakdown of continuum theory at the nanoscale is proposed—based on film morphology and chemistry from available experimental and molecular dynamics simulation data—to describe lubricant behavior under shear. Viscosity stiffening and interfacial slip models are introduced into the formulations of the normal (bearing) and shear forces acting on a sphere that moves within a thin lubricant film parallel to a rigid plane. The experimentally measured ‘apparent’ viscosity confounding the effects of both stiffening and slip is used to predict the hydrodynamic forces acting on a fully or partially submerged sphere for the purposes of describing lubricant contact in magnetic storage. The proposed sphere-on-flat model forms the basis of a future, dynamic contact with friction model that will account for lubricant contact in the context of molecularly thin lubricated rough surface contact.     

Some granular aspects of E.H.L. traction

A phenomenological examination of traction in elastohydrodynamic lubricated contacts shows there is a direct analogy with shear behaviour of beds of granules. The behaviour of a supposed granular lubricant under shear is formulated with regard to packing arrangements and agrees well with experimental results. The possibility of actual granularity in high-pressure lubricants is discussed and suggestions are made for methods of testing the hypothesis and for future work.     

Pressure viscosity coefficients and traction properties of synthetic lubricants for wind turbine gear systems

Synthetic lubricants are increasingly used to provide equipment reliability for wind turbine gear boxes. The majority of synthetic lubricants used today are based on polyalphaolefins. In gear systems where contact pressures are high, the pressure viscosity coefficient and traction values of the lubricant are important fundamental properties. A comparison of these properties for a wind turbine lubricant based on a polyalphaolefin and two lubricants based on polyalkylene glycols has been undertaken. Pressure viscosity coefficients were calculated from viscosity measurements made using an ultra‐high pressure falling needle viscometer at pressures up to 50 000 psi. Significant differences in properties were observed with both polyalkylene glycol lubricants showing lower pressure viscosity coefficients and much lower traction values. A calculation of the film thickness values in the Hertzian contact zone suggests that polyalkylene glycol lubricants may provide elastohydrodynamic films that are approximately 25% thicker than polyalphaolefin lubricants. Copyright © 2012 John Wiley & Sons, Ltd.     

Recent research on tribology in Japan

Recent research on tribology in Japan is outlined. There have been many papers on the following problems.

In fluid film lubrication, the effects of surface irregularities on the lubricating performance and increase in friction in high speed bearings due to turbulent flow are highlighted to date. Elastohydrodynamic lubrication and its related problems such as traction play an important role in the lubrication of cams and followers, rolling contact bearings and traction drives. The rheology of lubricants at high pressures is one of the most important properties governing lubricated concentrated contacts. Regarding basic studies on wear, the mechanism of the adhesive wear process is being studied. It is interesting to investigate the deformation behaviours of materials and structures by finite element methods. The friction and wear characteristics of plastics and their composites have been studied in basic terms.

The chemical reactions of lubricating oil additives with frictional surfaces play important roles in lubricated concentrated contacts. Recently there have been many studies on the correlation between the lubricating properties and the chemical reactivity of lubricant additives. It is very interesting that there are many papers on the mutual effects between lubricant additives.     

Effects of transverse atomic steps on bilayer lubricating films of simple hydrocarbons

Molecular dynamics (MD) simulations were conducted in order to study the dynamic behavior and traction of bilayer lubricating films of n-hexane, cyclohexane, and n-hexadecane. Lubricants were confined between bcc iron surfaces with and without transverse grooves of mono-atomic depth. Once the system equilibrated statically, one of the solid surfaces was moved to shear the film. The results demonstrated that the traction coefficient was governed by structures of the films, which depended on the molecular structures of the lubricants and on the atomic scale geometry of the solid surfaces. Traction was high when interfacial slip between lubricant layers and solid walls occurred. Evolution of the layered structure by gradual rearrangement of the molecules and resulting slip between the lubricant layers, caused significant reduction in the traction coefficient. The atomic steps enhanced the molecular rearrangement of n-hexadecane, while they retarded or inhibited those of n-hexane and cyclohexane resulting in a relatively higher traction coefficient for stepped surfaces. Molecular orientation of the normal alkanes under shear is described by the orientational order parameter, which has a strong correlation with the traction coefficient. The steady state traction coefficient of all the three simple hydrocarbons was highest when both of the surfaces had steps, and lowest when both of the surfaces were flat.