Entrainment Speed Research Articles

The Effect of Kinematic Conditions on Film Thickness in Compliant Lubricated Contact

The present paper deals with an investigation of film formation in compliant lubricated contact. Despite these contacts can be found in many applications of daily life including both biological and technical fields, so far little is known about the lubrication mechanisms inside the contacts. The main attention is paid to the effect of kinematic conditions on central film thickness. For this purpose, fluorescent microscopy method was employed. Experiments were realized in ball-on-disk configuration, while the ball was made from rubber and the disk was from optical glass. The contact was lubricated by glycerol and polyglycol to examine the effect of fluid viscosity. The measurements were conducted under pure rolling and rolling/sliding conditions. The entrainment speed varied from 10 to 400 mm/s and constant load of 0.2 N was applied. Experimental results were compared with two theoretical predictions derived for isoviscous-elastohydrodynamic lubrication (I-EHL) regime. It was found that the thickness of lubricating film gradually increases with increasing entrainment speed, which corresponds to theoretical assumptions. Against expectations, evident influence of slide-to-roll ratio (SRR) on film formation was observed. In the last part of the paper, some limitations of this study are discussed and several recommendations for further methodology improvement are suggested.

Elastohydrodynamic Lubrication with Oil Droplets

In oil–air lubrication, lubricants are supplied in the form of oil droplets to tribo-pairs. Numerical work has been carried out to investigate transient EHL characteristics when a series of oil droplets are entrained along the center line of the contact zone. Results show that the spacing distance between oil droplets is an important parameter for continuous EHL film formation. For oil droplets with the same volume, the effective lubrication time increases first and then decreases as the droplet distance increases. An optimum droplet distance can be found for the maximum effective lubrication time with continuous EHL films. Moreover, entrainment speed, lubricant viscosity and oil/solid interface can also affect the effective lubrication time significantly and change the optimum droplet distance accordingly. Numerical results showed a close correlation to the experiments. This study is useful to basic understanding of lubrication by limited lubricant supply.

Experimental study of elastohydrodynamic lubrication behaviour under single oil droplet supply

In this study, an oil droplet passing through an elastohydrodynamic lubrication contact is observed via interferometry, and the effects of droplet size, entrainment speed, load, and slide/roll ratio (SRR) are studied. The results show that the surface area of the droplet is enlarged because of the squeeze spread and capillary force at the inlet region, which influence the film growth. With a large droplet size, the spreading diameter increases and a thick film is generated. When the entrainment speed increases, the surface area enlargement of the oil droplet at the inlet is found to be insufficient, and only a local dimple-shaped film is formed. The SRR has a direct effect on the lubricant distribution. In particular, the load and viscosity effects exhibit different characteristics compared with the case of typical fully flooded lubrication.

Experimental investigation on the rheological properties of castor oil at different temperatures

Castor oil is one of the most enormous potential environment-friendly lubricants, and used as the base oil and additives in industrial applications. In this paper, the rheological property of castor oil was investigated using the optical interferometric test rig to analyze its flow properties under the micro gap. These results indicated that the central film thickness of castor oil in the point contact zone underwent the initial increased and then stable variation with the entrainment speed at different temperatures. The temperature and entrainment speed played an important role on the lubrication state and oil shape variation along the transverse and entrainment direction. At low temperature, the contact zone was almost in the thin lubrication regime at low entrainment speed, but was in the elastohydrodynamic lubrication regime at high entrainment speed. The side leakage went through the initial increased then decreased trend with the temperature and entrainment speed. These research results would further develop the engineering application of castor oil.

Effects of lateral harmonic vibrations on film thickness in EHL point contacts

Abstract Vibrations are an integral part of machine operation and affect every elastohydrodynamically lubricated conjunctions. In the present experimental study, a rolling contact is subjected to lateral-sliding vibrations with short stroke length. Film thickness behaviour is investigated under diverse conditions including rolling speeds, stroke lengths, frequencies, loads, and variety of lubricants. The results obtained under vibrations are compared with the results for corresponding steady state conditions to determine the effects of vibrations. It is shown that the lubricant rheology and the ratio of main and lateral entrainment speed are crucial parameters where the influence of this ratio on relative film thickness and its deviations can be estimated by exponential functions uniformly for all lubricants. The thresholds for film failures were found.

Experimental Observation of Cavitation Under Point and Elliptical Contacts with Fluid Film Lubrication

Cavitation in fluid film lubrication was experimentally observed by optical interferometry under point and elliptical contacts. Fern leaf cavities and stripe-shaped cavities presented themselves in pure rolling, while string-shaped cavities and sheet cavitation were found in disk simple sliding. Cavitation zone moves downstream toward the outlet of the contact zone with increasing entrainment speed in pure rolling, whereas it is just the reverse in disk simple sliding. Increasing applied load increases threshold speed of occurrence of sheet cavitation. Under elliptical contacts, this threshold speed increases with increasing included angle between the entrainment direction and the minor axis of the elliptical contact zone; the included angle mentioned above markedly affects the position and shape of cavitation zone.

Film Thickness and Friction Relationship in Grease Lubricated Rough Contacts

The relationship between the film generation and the coefficient of friction in grease lubricated contacts was investigated. Ball-on-disc tests were performed under different operating conditions: entrainment speed, lubricant temperature and surface roughness. The tests were performed with fully formulated greases and their base oils. The greases were formulated with different thickener types and also different base oils natures and viscosities. Film thickness measurements were performed in ball-on-glass disc tests, and Stribeck curves were measured in ball-on-steel disc tests with discs of different roughness. The role of the thickener and the base oil nature/viscosity on the film thickness and coefficient of friction was addressed and the greases’ performance was compared based on their formulation.

Numerical simulation of an oil droplet passing through an EHL contact

In oil-air lubrication lubricants are supplied by droplets to tribo-pairs. Numerical work has been carried out to investigate transient EHL film when an oil droplet is entrained. Results show that entrainment speed, oil viscosity and wettability of the oil/solid interface significantly affect the lubricant side flow across the entrainment direction (termed as squeeze spread) at the inlet, and consequently different EHL behaviors are presented. For fast speeds, high oil viscosity and low wettability, when a small droplet passes through the central contact a dimple-shaped film is generated and bounding surfaces cannot be fully separated. Numerical results showed close correlation to the experiments. This study is beneficial to basic understanding of lubrication by limited lubricant supply.

Suitability of ethyl cellulose as multifunctional additive for blends of vegetable oil-based lubricants

In a previous study, ethyl cellulose was successfully blended with castor oil and the results demonstrated its suitability to be used as additive to expand the range of operating conditions under which fluid film lubrication is sustained, mainly at high temperature. However, apparent solubility problems were detected when mixed with other vegetable oils with lower polarity than castor oil. In this work, a suitable combination of ethyl cellulose (EC) with both high oleic sunflower (HOSO) and castor (CO) oils was found able to reach stable and non gel-like blends with viscosities at 40 °C ranging between 62 and 493 cSt, and viscosity indexes fitting into group III (VI ≥ 120) of API classification of base fluids. The ternary HOSO/CO/EC blend showed an important reduction in friction coefficient at low entrainment speed, and generated a stable EHD-film at 100 °C of around 20 nm, which suggests better boundary properties than HOSO/EC or CO/EC blends. On the other hand, ethyl cellulose hindered wax crystallization process of these vegetable oil-based lubricants at 5 °C, yielding comparable results to those obtained with standard polymethacrylate backbone additives. Therefore, the suitable combination of both castor and HOSO with EC as multifunctional additive allow a set of eco-friendly base fluids to be formulated with a wide kinematic viscosity range, better viscosity-temperature dependence than many mineral or synthetic oils lubricants and excellent boundary lubrication properties, making them suitable for many lubricant applications.

Film formation and friction in grease lubricated rolling-sliding non-conformal contacts

This study investigates the film formation and friction in grease lubricated, rolling-sliding, non-conformal contacts over a range of entrainment speeds, surface roughnesses and contact temperatures. The effects of grease composition are assessed by employing custom made, additive free, lithium and diurea thickened greases, whose composition is systematically varied so that the effects of the thickener and the base oil can be isolated. All film thickness and friction measurements were conducted under fully-flooded conditions. It was found that at low speeds all tested greases are able to form thicker films than the corresponding base oils. The thickness and behaviour of these films is determined by the thickener type and is independent of the base oil viscosity and the test temperature. At higher speeds, the film thickness is governed by the base oil properties alone and can be predicted by the EHD theory. At low speeds, films with diurea greases grow with time under constant speed and residual films persist under load after contact motion ceases. The real lambda ratio, based on measured grease film thickness, was shown to correlate well with contact friction. The transition from the thickener dominated behaviour to that dominated by the base oil occurs at a relatively constant film thickness, regardless of the base oil viscosity and test temperatures, rather than at a given entrainment speed. Based on the presented evidence, it is here proposed that the mechanism of formation of grease films at low speeds, is analogous to that reported to operate in EHL contacts lubricated with colloidal dispersions, namely the mechanical entrapment and deposition of thickener fibres, and that, rather than the widely quoted ‘transition speed’, it is the ratio of the thickener fibre size to prevailing film thickness that determines the range of conditions under which the film enhancement due to the action of thickener is present.

Influence of double (w1/o/w2) emulsion composition on lubrication properties.

Double (w1/o/w2) emulsions are potential fat replacers in foods. Fats are known for their lubricating properties, which contribute to texture perception. It is therefore of interest to understand how the composition of double emulsions influences lubrication properties. This study focuses on the understanding of the influence of the fraction of inner dispersed aqueous phase w1 and the gelation of the w1 droplets on the lubrication properties of double emulsions. The addition of an inner water phase w1 to the oil droplets decreased friction at low entrainment speeds due to adsorption of the lipophilic emulsifier polyglycerol polyricinoleate (PGPR) at the hydrophobic tribo-surface. At higher entrainment speeds, double emulsions with w1 fractions of up to 6% (corresponding to fat reduction of 20%) displayed comparable tribological behavior as full-fat single (o/w2) emulsions. For double emulsions with gelled w1 droplets at higher w1 fractions of up to 15% (corresponding to fat reduction of 50%), an increase in friction was observed compared to full-fat single (o/w2) emulsions. The increase in friction is probably related to the presence of gelled droplets expelled from the inner w1 into the outer w2 phase, and to the deformability of (w1/o) droplets. Lubrication decreased when gelled particles were expelled from the inner w1 phase to the outer w2 phase. Lubrication also decreased when the deformability of (w1/o) droplets decreased, since less deformable (w1/o) droplets spread less easily on the tribo-pair surface. Knowledge about lubrication properties of double emulsions can be used in future studies to relate composition to sensory perception and develop double emulsions further as fat replacers.

Relating rheology and tribology of commercial dairy colloids to sensory perception.

This study aims to investigate the relationship between rheological and tribological properties of commercial full fat and fat-free/low fat versions of liquid and soft solid colloidal systems (milk, yoghurt, soft cream cheese) with their sensory properties. Oscillatory measurements (strain, frequency), flow curves and tribological measurements (lubrication behaviour using Stribeck analysis) were conducted. Oral condition was mimicked using artificial saliva at 37 ○C. Discrimination test was conducted by 63 untrained consumers, followed by a qualitative questionnaire. Consumers significantly discriminated the fat-free/low fat from the full fat versions (p < 0.01) in all product classes, with most common verbatim used being "creamy", "sweet" for the full fat versus "watery", "sour" for the fat-free samples. Flow behaviour of both versions of milk showed overlapping trends with no significant differences identified both in absence and presence of saliva (p > 0.05). Full fat and fat free yoghurts had similar yielding behaviour and elastic modulus (G'), even in simulated oral conditions. However, in case of soft cream cheese, the full fat version had a moderately higher G' than the low fat counterpart. Even in presence of artificial saliva, there was slight but significant difference in viscoelasticity between the cream cheese variants depending on fat content (p < 0.05). Stribeck curve analyses showed that at lower entrainment velocities (1-100 mm s-1), both full fat yoghurt and soft cream cheese exhibited a significantly lower traction coefficient when compared to fat-free/low fat versions (p < 0.05), which might be attributed to the lubricating effect of the coalesced fat droplets. Surprisingly, whole and skim milks showed no significant difference in traction coefficients irrespective of the entrainment speeds (p > 0.05). Results suggest that sensory distinction between fat-free and full fat versions, particularly in semi-solid systems could be better predicted by lubrication data as compared to bulk rheology.

Behaviors of a micro oil droplet in an EHL contact

Oil–air lubrication supplies lubricants in the form of droplets to elastohydrodynamic lubrication (EHL) contacts, such as those in high-speed spindle bearings. However, there is a paucity of information related to understanding the lubrication behaviors of oil droplets within EHL contacts. In this study, behaviors of lubricant droplets, in terms of spreading around a static contact as well as passing through a rolling contact, were studied with an optical ball-on-disk EHL test rig. Influences of oil droplet size, viscosity, and surface tension on droplet spreading were examined. Lubricating film formation was also investigated when droplets traveled through the EHL contact region. The results indicated that droplet size and running speed significantly influenced film profiles. With increasing entrainment speeds, a small droplet passed through the contact without spreading and generated films with a significant depression in the central contact region.

Investigation of Shear-Thinning Behavior on Film Thickness and Friction Coefficient of Polyalphaolefin Base Fluids With Varying Olefin Copolymer Content

This study investigates the rheological properties, elastohydrodynamic (EHD) film-forming capability, and friction coefficients of low molecular mass poly-α-olefin (PAO) base stocks with varying contents of high molecular mass olefin copolymers (OCPs) to assess their shear stability and their potential for energy-efficient lubrication. Several PAO–OCP mixtures were blended in order to examine the relationship between their additive content and tribological performance. Gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) spectroscopy were used to characterize the molecular masses and structures, respectively. Density, viscosity, EHD film thickness, and friction were measured at 303 K, 348 K, and 398 K. Film thickness and friction were studied at entrainment speeds relevant to the boundary, mixed, and full-film lubrication regimes. The PAO–OCP mixtures underwent temporary shear-thinning resulting in decreases in film thickness and hydrodynamic friction. These results demonstrate that the shear characteristics of PAO–OCP mixtures can be tuned with the OCP content and provide insight into the effects of additives on EHD characteristics.

Experimental and Numerical Investigation of Inlet Dimple in Point and Elliptical Contact Elastohydrodynamic Lubrication Films

The experimental observation of the inlet dimple of the elastohydrodynamic lubrication (EHL) film was carried out in point and elliptical contacts, respectively, and then, the numerical analysis under point contacts was conducted. The inlet dimple appears and then moves upstream toward the inlet and finally disappears with the increase of the entrainment speed. In the meantime, the dimple depth increases at the beginning and then decreases. The high load leads to a wide entrainment speed range where dimple exists. The varying range of the entrainment speed corresponding to the dimple appearance is smaller at a smaller included angle between the minor axis of elliptical contact and the entrainment direction.

Transition from plug-flow to linear speed profile near a dent in a rolling–sliding EHL contact

The paper presents measurements of perturbed film variations during the passage across an EHL rolling–sliding contact. Perturbed film was produced by indentation on a ball. The data for increasing entrainment speed showed the change from steady to variable profile. A simple simulation of a film profile has provided an estimate of the through-film speed profile. The measured data can be explained by a transition from the plug-flow to the linear speed profile. It was found that the transition is occurring at different speed for two sliding cases. Two possible effects connected with an indentation impact on in-contact conditions were discussed.

Film thickness and friction behaviour of thermally aged lubricating greases

Non-additized batches of lubricating greases were subjected to an artificial thermal aging procedure. The fresh and aged samples were then tribologically characterized through film thickness measurements and Stribeck curves, over a large range of entrainment speeds on a ball-on-disc test rig at different operating temperatures, ensuring fully flooded conditions. The results were analysed at the light of the physical and chemical changes regarding the fresh greases.The film thickness increased for all greases after the thermal aging. The film thickness plateau at low speeds also changed and for certain greases it disappeared under the same range of entrainment speeds. The friction behaviour also changed considerably given the changes to the lubrication regimes for the same speeds.

Experimental measuring set‐up and procedure for the traction coefficient on roller‐on‐disc contact

AbstractA pin‐on‐disc machine was modified to measure traction coefficient and the potential of contact difference under controlled conditions of slide‐to‐roll ratio, temperature, contact pressure and entrainment speed; oil, grease, grease's base oil and bleed‐oil were tested. This new arrangement proved to be efficient to characterise lubricants traction behaviour, film separation and tribofilm generation. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Tribological method to measure lubricating properties of dairy products

Tribology has a growing interest in oral texture and sensory research due to its ability to assess certain properties of the food during the complex oral processing that cannot be explained by its bulk texture and rheology. Developing a reliable, low cost and easy-to-use tribometer applicable to a wide variety of food products is still a big challenge to researchers. In this work, a simple method to measure lubricating properties (friction coefficient) of dairy products is presented using a newly introduced tribometer coupled with a widely used rheometer. Pasteurized milks (fat contents from 0.1% to 4.9%) and cream cheeses (fats content: 0.5%, 5.5%, 11.6%) were chosen as representative dairy products and their friction coefficients were measured as a function of entrainment speed of the tribometer. The friction coefficients of the samples at low entrainment speed generating low shear rate (similar to the shear rate in mouth) were significantly different between the samples at each fat levels. Thus, this method is capable of differentiating samples with different fat contents both in liquid or semi-solid forms. This suggests a promising application of this technique for a quick assessment of the sensory mouthfeel of various dairy products in relation to fat content.

Correlation of Polysiloxane Molecular Structure to Shear-Thinning Power-Law Exponent Using Elastohydrodynamic Film Thickness Measurements

Siloxane-based polymers (polysiloxanes) are susceptible to temporary shear-thinning that manifests as a reduction of elastohydrodynamic film thickness with increasing entrainment speed or effective shear rate. The departure from Newtonian film thickness can be predicted with the power-law exponent ns, an indicator of the severity of shear-thinning in a polymeric fluid that is influenced by the macromolecular structure. In this paper, a combination of extant rheological and tribological models is applied to determine the power-law exponent of several polysiloxanes using film thickness measurements. Film thickness data at several temperatures and slide-to-roll ratios are used to validate the methodology for several siloxane-based polymers with alkyl and aryl branches.