Optical Elastohydrodynamic Lubrication Research Articles

Surface plasmon resonance microscopy for optical EHL measurement systems

This study investigates the suitability and limitations of surface plasmon resonance (SPR) microscopy for optical elastohydrodynamic lubrication (EHL) measurement systems. SPR microscopy developed in this study can quantify the lubricant density distribution in contact region with high spatial-temporal resolution using simple total reflection optical systems. It means that, if either the temperature or pressure distribution can be estimated, the other can be calculated. To examine the practicalities, we determined specific optical setting conditions such as the incident angle and thickness of each film layer for surface plasmon excitations under EHL conditions based on numerical calculations and demonstration experiments.

Experimental and Theoretical Study on Forming Film Properties of Hollow Cylindrical Rollers

Bending deformation of a hollow cylindrical roller (HCR) was calculated by curve bending theory and slice technology. Furthermore, an elastohydrodynamic lubrication (EHL) model involving hollow rollers was established by coupling the finite line EHL model and bending deformation. Measurements of film thickness for hollow rollers were carried out by using a line-contact optical EHL test rig, and the experimental data and theoretical results agreed well with each other. For a roller with a straight-line profile, the hollow structure could reduce pressure peak at the roller ends, but the edge effect was still present. The film thickness at the ends unfortunately could not be improved by such hollow structures, unless a profile modification for the operating surface is designed.

Experimental Study on the Influence of Stearic Acid Additive on the Elastohydrodynamic Lubrication of Mineral Oil 2137

Using an optical elastohydrodynamic lubrication (EHL) test rig, oil film thickness and the coefficient of friction (COF) were measured, and the influence of stearic acid additive on the EHL performance of mineral oil 2137 was investigated. The results showed that 2137 with 0.3 wt% stearic acid (denoted to as 2137s) achieved the same film thickness as 2137, while the COF of 2137s was significantly lower than that of 2137 when the contact was under conditions of a fully lubricant supply. Under conditions of limited lubricant supply, 2137 base oil was prone to oil starvation with the increase of entrainment velocity. On the other hand, 2137s significantly mitigated the oil starvation. This was attributed to the fact that lower surface energy by the adsorption of stearic acid results in discontinuous oil-droplet distribution on the lubrication track and, therefore, early pressure generation. Moreover, it is interesting to find that less 2137s supply quantity can produce higher film thickness when the contact is at high speeds, which is attributed to the fact that a smaller quantity of 2137s gives smaller droplets on the lubrication track, and the resultant small surface area–volume ratio presents oil more resistance to the centrifugal force and results in less oil escaping from the lubrication track. The addition of stearic acid reduced the average COF of 2137 mineral oil by about 13.3%

Influence of Metal Working Fluids in Cutting Processes

For the realization of efficient production processes, an understanding of the appropriate application of metal working fluids (MWF) is necessary. In addition to knowledge about the process-related aspect of chip transport and the macroscopic cooling effect, the characteristics and properties of the lubrication film thickness and the cooling conditions in the area of the secondary shear zone on the chip surface, i.e. in the direct vicinity of the material separation, represent a fundamental scientific issue within production technology. In particular, these areas generate a high proportion of heat during machining, so that the local friction phenomena have a significant influence on the resulting edge zone of the produced component and the thermomechanical load on the tool. Currently, there are no numerical models and methods for mapping and predicting the lubrication film thickness that can be used in the sense of a targeted design of the cooling lubricant supply. The aim is to transfer the methods from the field of tribology of machine elements, which have already led to significant knowledge gains in this discipline, to machining and couple them to approaches already established in machining. To this end, experiments on tribometers have been performed as a first step. For example, an oscillating pin-on-plate tribometer was used. In this setup, a steel plate is doing oscillating motion against a fixed ball (diameter of 6 mm) under a defined load. The frictional force is recorded during the test. A MWF in a heated tank is used for the lubricant. Additional investigations on the film thickness were performed on an optical EHL (elasto-hydrodynamic lubrication) tribometer. In this setup, a ball rolls on a glass-disc and the resulting film thickness is measured by interferometry.For comparison, the influence of the MWF on the chip formation process in metal cutting was investigated on a special test rig (machine tool). This test rig allows high speed imaging and force measurements of an orthogonal cutting process while using MWFs. The first results show a reduced contact length between chip and tool as well as lower process forces for processes with MWFs compared to dry cutting processes. In future investigations, this test rig will be applied for the identification of the local friction coefficient between chip and tool. The data gained from the cutting test are compared with the output of the tribological test rigs.

Your EHD Rig May Not Be As Elastohydrodynamic As You Think

Abstract The concentrated contact formed between a steel ball and a glass disc—the optical elastohydrodynamic lubrication (EHD) rig—has been the primary instrument for experimental investigations of elastohydrodynamic film thickness. It has been a source for values of pressure-viscosity coefficient, a difficult-to-define property of liquids. However, comparisons with the pressure dependence of the viscosity obtained in viscometers show little agreement. There are multiple reasons for this failure including shear-thinning and compressibility of the oil. Another reason for the poor agreement is the subject of this short note. The optical EHD rig using glass as one surface will only be in the piezoviscous-elastic (EHD) regime when the pressure-viscosity coefficient is large. For low values, it would be operating in the isoviscous-elastic regime (soft EHD).

Study on the free edge effect on finite line contact elastohydrodynamic lubrication

Abstract This paper incorporated a recently developed algorithm, which directly gives the elastic deformation of loaded quarter-space (QS) bodies, into a finite line contact elastohydrodynamic lubrication (EHL) analysis. Half-space (HS) and QS models were adopted in the analysis and the results were compared with the data obtained from conventional optical EHL tests of steel rollers on a glass disc. This study found that contact deformation was dominated by glass disc deformation. Thus, the use of a steel roller on a glass disc in conventional optical EHL technique is not appropriate for studying the free edge surface effect. Free edge surfaces reduce the magnitude of pressure spikes near the ends of contact. Rollers with different end profiles were also evaluated.

Effects of Load, Squeeze Velocity, Viscosity on Pure Squeeze EHL Motion Using Optical Interferometry

This paper presents a novel experiment method to investigate the microscopic mechanism of the oil film under the pure squeeze elastohydrodynamic lubrication (EHL) motion. An optical EHL squeeze tester is used to measure the interference fringe patterns of the contact region. In order to show the dimple thickness clearly, the grayscale interferometry method is employed to obtain the film thickness map. In addition, the effects of squeeze speed, load, and lubricant viscosity on the dimple film thickness are explored under a quasi-static condition.

Effect of Interfacial Properties on EHL Under Pure Sliding Conditions

This paper presents an experimental study of the effect of boundary slip on the lubricating film shape and friction of an elastohydrodynamic lubrication (EHL) contact under isothermal conditions. Ball and disc pure sliding experiments were carried out with a high viscosity polybutene oil using a conventional optical EHL test rig. The film shape and friction were measured simultaneously. The results obtained from two discs with different coatings were compared. One disc was coated only with Cr, the partially reflective layer, and the other had an extra layer of SiO2 coating on top. When running under mild conditions of low load and speed, there was no evidence of any boundary slip effect. However, when the load increased, the Cr-coated disc produced lower film thickness and friction than the SiO2-coated disc. The Cr-coated surface had a larger contact angle, i.e., smaller surface energy, than the SiO2 surface, which reflects the weak bonding between the molecules of the surface and the lubricant. The study concludes that surfaces with low surface energy promote boundary slip at the EHL contact, leading to a reduction in friction and film thickness.

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.

Effects of temperature on pure squeeze EHL motion using grayscale interferometric method

Purpose – The purpose of this paper is to present a novel method to investigate the microscopic mechanism of the oil film under the pure squeeze elastohydrodynamic lubrication (EHL) motion. An optical EHL squeeze tester is used to explore the effects of squeeze velocity, load, temperature, and lubricant viscosity on the dimple film thickness that occurs when a ball approaches a flat plate covered by a thin layer of oil.Design/methodology/approach – The grayscale interferometric technique was used to study the thickness of the lubricating film in an EHL point contact. The light source was a He‐Ne laser. Through the transparent optical glass and by means of optical interference, the interference fringe patterns of the contact region were observed by a charge‐coupled device camera recording. The two elastic bodies were a sapphire disk and a steel ball. The contact was lubricated with paraffin‐based oil.Findings – Results show that increasing the squeeze speed, load, viscosity, and decreasing the temperature,...

Occurrence of Wall Slip in Elastohydrodynamic Lubrication Contacts

Preliminary experimental work has been carried out to identify some of the boundary slip phenomena of highly pressurised polybutenes in an elastohydrodynamic lubrication (EHL) conjunction. The movement of the oil is signified using an entrapment that can be readily formed by the impact of a steel ball against a layer of oil on a glass block in an optical EHL test apparatus. The post-impact lateral movement of the entrapment was investigated under the conditions: (i) pure rolling, (ii) pure glass block sliding (steel ball stationary) and (iii) pure ball sliding (glass block stationary). It was observed that under pure rolling the entrapped oil travels within the contact region at the entrainment speed, which is correlated with EHL theory. Under pure glass block sliding conditions, the speed of the entrapped oil core is less than the entrainment speed, and in the extreme cases, this core can be nearly stationary. Under pure ball sliding conditions, the oil core moves at a speed greater than the entrainment speed. The observation indicates that the oil/steel ball interface can sustain higher shear stress than the oil/glass (chromium coated) interface and there is a boundary slip in terms of relative sliding at the latter interface under the experimental conditions. Furthermore, the amount of slip increases with an increase in the pressure. These experiments provide evidence of the existence of wall slippage, which leads to the abnormal EHL film profile characterised with an inlet dimple as reported earlier.

Theoretical Study on the Interferometry of Thin EHL Film Measurement

Interferometry has been widely utilized in elastohydrodynamic lubrication (EHL) film measurement since the 1960s. In-depth optical analyses are required to get more fundamentals of optical EHL film tests. In this paper, a stratified-layer model of optical EHL contact is numerically analyzed by the multi-beam interference approach. It is revealed that the high reflectance of the beam-splitter and the steel interface generate a deviation of the intensity profile from the two-beam interference. Previous experimental results about fringe contrast are reproduced, and the determination of the beam-splitter layer is theoretically clarified. Furthermore, some characteristics of spectrometry for ultra-thin film measurement are studied, and the dependence of the interference spectrum on the spacer layer thickness is displayed. The measurement simulation has theoretically confirmed the constant initial phase change assumed in practical measurement. It is theoretically demonstrated that in the spectrometry approach, TiO2 layer can generate spectrum with high finesse and may be used in the future application. The theoretical results in this paper are correlated with previous experimental practices, and the optical EHL technique can therefore be much better theoretically understood.

Experimental study on lubrication regime variation in point contacts

The change between elastohydrodynamic lubrication (EHL) and hydrodynamic lubrication (HL) under a wide range of entrainment speeds and applied loads was studied using an optical EHL apparatus. A log–log scale linear relationship was demonstrated in the two lubrication regions between the film thickness and the entrainment speed (or load). A transition region can be clearly discerned between these two regions in which the film thickness no longer bears a linear relationship with the entrainment speed (or load). It is shown that a piezoviscous effect can be distinguished in the HL region by the speed exponent or the load exponent, and that relative sliding has a significant influence on the transition region.

Inverse approach for estimating rheological characteristics of adsorption layer in thin film EHL contacts

A modified Reynolds equation is derived for thin film elastohydrodynamic lubrication (TFEHL) by means of the viscous adsorption theory. This TFEHL theory can be used to explain the deviation between the measured film thickness and that predicted from the convenient elastohydrodynamic lubrication (EHL) theory under very thin film conditions. Results show that the thinner the film, the greater the ratio of the adsorption layer to the total film thickness becomes, and the greater the value of the pressure–viscosity index ( z′). An inverse approach is proposed to estimate the pressure distribution based upon the film thickness measurement and to determine the pressure–viscosity index of oil film, and the thickness ( δ) and the viscosity ratio ( η *) of the adsorption layer in TFEHL circular contacts. Based on TFEHL theory, the inverse approach can reduce z′ error, and provides a reasonably smooth curve of pressure profile by implementing the measurement error in the film thickness. This algorithm not only estimates the pressure, but also calibrates the film shape. Consequently, it predicts z′, η *, and δ with very good accuracy. It can also be used to evaluate the lubrication performance from a film thickness map obtained from an optical EHL tester. Results show that the estimated value of z′ is in very good agreement with the experimental data.

Study on pure squeeze elastohydrodynamic lubrication motion using optical interferometry and the inverse approach

This paper uses an optical elastohydrodynamic lubrication (EHL) impact tester to explore the effects of squeeze velocity, load, and lubricant viscosity on the dimple film thickness occurring when a ball impacts a flat plate covered by a thin layer of oil. Three types of motion for the ball are combined and simulated in this experiment to find important operation parameters. In the direct numerical solution, hydrodynamic lubrication is assumed at the initial impact stage and EHL is assumed at the high-pressure stage. Pressure and dimple shapes are obtained by numerical solution of the coupled transient Reynolds equation, elasticity equation, and ball motion equation. The inverse approach is proposed to estimate pressure and apparent viscosity in pure squeeze EHL motion. Results show that, with higher impact speeds and loads, more time is needed to reach stability. Also with lower viscosities, more time is needed to reach stability. Increasing the squeeze speed, load, and viscosity all make the dimple deeper. The post-impact high-pressure condition for the constant-load case is suitable for evaluating the pressure-viscosity index. The inverse approach is successfully used to estimate the pressure distribution resulting from the film thickness map obtained by interferometry. Agreement between the experimental value of the test fluid and the estimated value is quite good.

Inverse approach for estimating pressure and viscosity in the elastohydrodynamic lubrication of circular contacts

The film thickness under steady state conditions can be measured by using an optical interferometer. An inverse approach is proposed for estimating the pressure distribution on the basis of film thickness measurement in elastohydrodynamic lubrication (EHL) circular contacts. This approach is constructed from the approximated model of elastic deformation and force balance equations. To obtain an accurate pressure, it is necessary to divide the domain into a few regions on account of the singularity at the pressure spike. The principle of measuring point selection is proposed, and the problem of pressure fluctuation is overcome. On the basis of the smoothed pressure distribution, the apparent viscosity of the film can be obtained from the Reynolds equation. The least-squares method is used to compute the optimum value of the pressure-viscosity index. Results show that the best region for estimating the pressure-viscosity index is along the x axis because the Poiseuille term becomes zero in the Reynolds equation on account of the symmetry. In this region, the estimated pressure-viscosity index shows very good agreement with the exact value when measurement errors are neglected. When measurement errors are taken into account, the close agreement shows the potential of the proposed approach in estimating accurate values of the pressure-viscosity index. Generally, the error in estimating the pressure-viscosity index increases with increasing standard deviation of the measurement error, load, speed, material parameter and absolute error of the measured film thickness. The inverse approach can also be used to estimate the pressure distribution on a film thickness map obtained from an optical EHL tester. Moreover, the agreement between the actual and the estimated values of z is quite good.

The measurement of the refractive index of a liquid lubricant at high pressure within an EHL impact dimple using a single optical interferogram

The optical film thickness, expressed as the product of the refractive index and the absolute film thickness, is often considered as a single parameter in optical elastohydrodynamic lubrication (EHL) studies. A theoretical study of the effect of refractive index upon the interference intensity has been carried out, providing a method for the evaluation of refractive index irrespective of the absolute film thickness. As a result, a simple and reliable method for measuring the refractive index of an oil in an optical EHL contact has been developed. The technique is applied to a highly pressurized liquid lubricant trapped inside an EHL dimple and its high pressure refractive index is determined.

A multi-beam intensity-based approach for lubricant film measurements in non-conformal contacts

Studies of elastohydrodynamic lubrication (EHL) frequently utilize the phenomenon of optical interference which is exhibited when an illuminated steel ball is in contact with a partially Cr-coated glass disc in conjunction with a lubricant layer of known refractive index. The use of the steel and chromium surfaces enables lubricants having similar refractive indices to glass to be examined. The EHL contact is composed of two microscopically thin layers: the Cr coating and the lubricant film, bounded by the glass disc and the steel ball. That the two-beam interference method frequently used in optical EHL research may induce significant errors in the measurement of film thickness is reported in this paper. A theoretical analysis which includes consideration of the effects of both multi-beam interference and optical absorption within the Cr film and the steel surface is presented. The analysis is used to devise a multi-beam intensity-based test technique which has been validated for a wide range of lubricant film thickness measurements at a resolution of 1 nm and offers a practical minimum detectable film thickness as thin as 1 nm.