Hydrodynamic Friction Research Articles

Characterizing friction for fiber reinforced composites manufacturing: Method development and effect of process parameters

In automated layup manufacturing processes of fiber-reinforced polymer composites, the quality of the manufactured part is strongly dependent on frictional behavior. Improper control of frictional forces can lead to defect formation. Frictional sliding rheometry tests provide an innovative methodology to accurately characterize the tool-ply friction of unidirectional (UD) prepreg employing unique annular plate geometries. The effect of processing parameters (temperature, velocity, and normal force) on the frictional response of a carbon fiber prepreg was studied. Moreover, utilizing custom designed plate geometries coupled with optically transparent fixtures allowed for in-situ quantification of the prepreg-rigid surface contact area along with simultaneous characterization of the process parameter-dependent frictional mechanisms. Our findings highlight the reduction in frictional forces with increasing temperature, attributed to the increased resin flowability, while increases in sliding rates resulted in a pronounced increase in the frictional forces. The effect of applied load on the frictional characteristics was more complicated due to contributions from both the adhesive and normal forces. Finally, the results were interpreted in light of the contact area measurements performed at different temperatures, normal force, and sliding rate. • Frictional sliding properties of carbon fiber prepregs are characterized with an annular plate geometry in a rheometer. • Effect of processing variables (temperature, sliding velocity, and normal force) on the frictional properties of the prepreg is investigated. • Optical microscopy facilitates quantification of the contact area between the prepreg and rigid substrate. • Frictional sliding behavior transitions from mixed to hydrodynamic lubrication regime of a Stribeck curve. • Frictional forces are controlled by both adhesive forces and applied load, leading to deviations from Amontons' law.

Formulation of lyotropic liquid crystal emulsion based on natural sucrose ester and its tribological behavior as novel lubricant

The tribological behavior of oil-in-water emulsions formulated with natural lyotropic liquid crystal (LLC) emulsifiers based on natural sucrose ester was studied for the first time. Polarized optical microscopy, synchrotron radiation small-angle X-ray scattering, wide-angle X-ray scattering, and synchrotron radiation infrared microspectroscopy demonstrated that LLC emulsifiers were tightly ordered at the oil—water interface with a distinct nematic texture. The viscosity of emulsion was observed to change over time. Moreover, the zeta potential and laser particle size distribution verified the emulsion’s satisfactory stability. The frictional shearing test proved that the coefficient of friction of the emulsion versus pure oil decreased by 34.2%. The coefficient of friction of the emulsion with liquid crystal decreased 10.1% versus that without liquid crystal. Although liquid crystal emulsion did not exhibit outstanding anti-wear performance compared with pure oil, its wear volume was 29.4% less than the emulsion without liquid crystal. X-ray photoelectron spectroscopy and scanning electron microscope—energy dispersive X-ray spectroscopy (SEM—EDS) proved that the tribo-film of the emulsion with liquid crystal was formed synergistically by the liquid crystal phase with the base oil. The formulation affecting the lubricant quality was further studied by orthogonal experiments. The resulting Stribeck curve behavior suggested that proper composition with a slightly higher viscosity can better reduce friction in both boundary lubrication and mixed lubrication regimes. The lubrication mechanism indicated that the periodically ordered liquid crystal was transported to the sliding asperity in the form of emulsion droplets, which bored the pressure and released the oil to form a tribo-film. This LLC emulsion is environmentally friendly and potentially non-irritant to the skin. Thus, it has promising application prospects as novel water-based and biological lubricants.

Enhancement of hydrodynamic friction by periodic variation of contact stiffness

A common way to increase friction during lubricated sliding is to increase energy dissipation in the slider or the substrate, for example, by bulk viscoelasticity. It has recently been shown that lubricated friction can also be enhanced by surface architecture, specifically by periodic variation of near-surface stiffness. We study this phenomenon by considering a rigid cylinder undergoing lubricated sliding on a substrate with a periodic variation in mechanical stiffness. We model the process using the Reynolds theory for transient elasto-hydrodynamic lubrication (t-EHL). We developed a numerical scheme to solve this t-EHL problem and used it to study how surface variation in stiffness affects the motion of the cylinder, the distribution of hydrodynamic pressure, the liquid film thickness, and the friction force. Our results indicate that increasing the variation of the near-surface stiffness can significantly increase the friction force. The numerical method developed in this work can be applied to other transient EHL problems with slight modification. Our results provide insight into the mechanism of friction enhancement and can guide design of surfaces to control friction during lubricated sliding.

Influence of magnetic property of test plates on magneto-rheological behavior

This paper investigated the behaviors of magnetorheological fluids (MRFs) under shear mode with magnetic iron and non-magnetic titanium upper plates, respectively. Based on the Stribeck curve and friction coefficient normalization methods, the lubrication states at the chain end/plate contacts were more at a boundary lubrication state. The restriction of magnetic plate on the particles ends migrations under high magnetic field enhanced the field induced friction to increase the yield stress. Therefore magnetic plate prevented sliding of particles with plate and resulted in a higher yield stress than the easier sliding of particle chains ends on non-magnetic plate. The magnetic property of the upper test plate should be properly considered during the tests of MRFs to derive accurate description of the properties of MRF to be used in the design of various MR actuators.

Theoretical research of couple stress effect and surface roughness on lubrication regimes transition of misaligned hydrodynamic journal

The purpose of this paper was to investigate the combined effects of couple stress and surface roughness on lubrication performance and regime transition of misaligned hydrodynamic journals. New lubrication models considering misalignment angle, couple stress, and surface roughness were established. Under the multi-coupled factors, Stribeck curves were exploited to reveal the lubrication regimes of journals. Following Greenwood’s aspect contact theory, force relational equations among external load, asperity support capacity, and oil film bearing capacity have been further discussed. Results showed that the growth of the minimum film thickness, the maximum film pressure, and friction coefficient versus couple stress parameters trended to be opposite to those versus misalignment angle. The speed of the lubrication regime transition and the minimum friction coefficient increased with the enlargement of the misalignment angle but decreased as the couple stress parameter increased. Higher couple stress parameters resulted in an increment of the safety speed of the journal and were conducive to maintaining the hydrodynamic lubrication regime of the misaligned bearing at low speed, which is applicable to such misaligned journal bearings in practical engineering.

Effect of chymosin‐induced hydrolysis of αS1‐casein on the tribological behaviour of brine‐salted semihard cheeses

This study investigated the effect of chymosin‐induced hydrolysis of αS1‐casein on the tribological behaviour of cheese. Residual chymosin activity was inhibited by adding a chymosin inhibitor (i.e. pepstatin A) to the curd–whey mixture during cheese manufacture. A significant difference was observed in the static and kinetic regimes of the extended Stribeck curves of cheese samples, suggesting that the chymosin‐induced hydrolysis of αS1‐casein affects the friction and lubrication properties of cheese, which may be relevant to the mouthfeel‐related sensory attributes of the cheese. Overall, the modulation of hydrolysis of αS1‐casein could be an approach to alter the friction‐related sensory attributes of cheese.

Model of Hydrodynamic Friction in the Boundary Layer on an Inclined Flat Surface of an Aircraft

An integral method for solving the problem of longitudinal flow of an inclined flat plate located in a stream of air flow is presented. As a result of the integration of the equations of motion over the thickness of the boundary layer, the two-dimensional problem becomes one-dimensional. The problem was solved numerically in the Mathcad software package. The graphs of the dependence of the thickness of the boundary layer and friction on the longitudinal coordinate at different angles of inclination of the plate are presented. The flow mathematical model can be used to determine the characteristics of two-dimensional boundary layers with arbitrary boundary conditions.

Tribology and the Ocular Surface.

IntroductionTribology is known as the science of friction, lubrication and the determination of what occurs when two surfaces slide against one another. The required partners in this science are a minimum of two surfaces and relative motion. Tribology could be a key factor in dissecting the issues that surround and confound dry eye in patients as well as contact lens discomfort and intolerance. Specifically, it is this issue that is a potential causative underlying factor that could lead to conditions like lid wiper epitheliopathy (LWE).MethodsPeer-reviewed literature was reviewed as It pertains to ocular tribology and the application to the ocular anatomy. A PubMed review was performed using the keywords: tribology, friction, lid wiper epitheliopathy, contact lens, and dry eye. All manuscripts were reviewed for mentions of tribology and friction. The exact same process was performed with The Association for Research in Vision and Ophthalmology (ARVO) abstracts.ResultsIn relation to the ocular surface, tribology describes the mechanical interactions between the upper and lower lid wipers and the globe. The Stribeck curve can be applied, as the sliding partners are separated by a complex, lubricating tear film. The surface brush anatomy at the eye to eyelid juncture reduces friction and alters the Stribeck curve in favor of the hydrodynamic regime, allowing for high velocity movement with minimal wear. Changes to the tear film or the introduction of a contact lens can displace the Stribeck curve, increase friction, and induce wear thus leading to patient symptomology.ConclusionFurther studies may provide new insight into contact lens discomfort and ocular surface disease, including LWE; however, adaptation of tribology work performed in vitro to in vivo patient care is challenging.

Dependency of the mobility of carboxymethylated cellulosic polyions on the nature of their counterions

In this paper, we propose a new approach in order to interpret the dependency of the conductibility of carboxymethylated cellulosic polyelectrolytes XCMC1, XCMC2, XCMC3 in water at 25 °C, with their alkali counterions (X ≡ Li+, Na+ and K+) or ammonium counterions (X ≡ Et4N+ and Bu4N+).The 750 ring-blocks of each CMCi chain are randomly substituted by CH2COO groups, of electric charge (eαX), where αX is the degree of dissociation of the counterions X. It is found that for a given CX ≤ 7 10−3 mol·l−1, the experimental equivalent conductivity λP,X of the polyion CMCi decreases as follows: λP, K+ > λP, Na+ ≥ λP,Li+ > λP,Et4N+ > λP,Bu4N+. This behavior cannot be explained either by a specific ionic condensation, or in terms of hydrodynamic friction or ionic friction undergone by the polyion, or in terms of change of configuration of the polyions (ellipsoidal configuration). However, calculations taking into account the dielectric friction effect according to our new general formula, recently published, giving the dielectric friction coefficient, βPdf, for pearl-necklace-like polyelectrolyte chains, allowed to obtain good convergence between experimental and theoretical results. This new approach improves the interpretations given by Manning and Vink in terms of electrophoretic effect only.

An experimental investigation of low viscosity lubricants on three piece oil control rings cylinder liner friction

Reducing energy use and improving engine efficiency is a complex task however to date a continued reduction of lubricant viscosity has proved effective. Reducing viscosity decreases hydrodynamic friction and pumping losses however it can also exacerbate boundary friction losses. Detailed and representative component level experimentation is required to understand the effects of viscosity reduction on friction and opportunity for further optimisation. This paper presents a novel motored reciprocating Tribometer which has been developed to measure the friction between complete cylinder liners and three-piece oil control rings. The system holds individual or multiple rings stationary in a bespoke ring holder and reciprocate the cylinder liner thereby replicates the relative kinematics of the components in service. The new design has many operational advantages to identify and benchmark the individual contribution of oil control ring friction including near total isolation of oil ring-cylinder liner bore conjunction, pure rectilinear motion and use of full components without resorting to split liner/ring geometries. The experimental rig is used to measure friction at the three-piece oil control ring- cylinder liner conjunction when lubricated with two low viscosity lubricants. The results show prevalence of mixed regime lubrication across the speeds, temperatures and lubricants investigated. The oil control ring under investigation is shown to operate in mixed regime lubrication and at cold start the introduction of lower viscosity lubricants such as 0W-8 showed higher level of oil control ring-cylinder liner friction in comparison to the 0W-40. The information and experimental facility are of critical use for engine designers when considering the potential contradictory component efficiency behaviour when moving to ultra-low viscosity lubricants.

Hydrodynamic effect of technological lubrication and friction modes formation at sheet rolling

One of the main engineering industries is the processing of metals by pressure. Its purpose is the formation of products from metal blanks of simple forms. An important role in this case belongs to the technology of sheet rolling. The solution of issues related to the choice of optimal modes of sheet rolling is relevant for specialists in the field under consideration. The process of rolling metal products is special, since the contact stresses can exceed the deformation resistance of rolled metal by several times. The tasks of predicting the quality of products, taking into account the prevention of sheet metal continuity defects, as well as the study of technological operations reliability of rolling process are associated with the solution of industrial and scientific problems in this area. Domestic and foreign researchers have noted a significant influence of external friction in the deformation center on the technological parameters of rolling process and quality of the resulting metal products. It is known that rolling is impossible without external friction between metal and roll. The reduction of friction provides an opportunity to increase the economic indicators of sheet rolling, and the rolling production technology itself has significant opportunities for further improvement. Technology of sheet rolling in the conditions of hydrodynamic friction mode allows modeling of friction conditions in the deformation zone. Taking into account the hydrodynamic effect of lubrication allows us to assess the influence of technological factors on the conditions of contact friction during metal pressure treatment and to control the rolling process. Regularities of the flow of liquid Newtonian lubricant are investigated, taking into account features of the deformation zone geometry during sheet rolling. Pumping capacity of the lubricating wedge during rolling is determined, which allows us to explain the regularities of lubrication influence on friction conditions. As an example, the results of calculation of the lubricant pressure depending on the rolling speed are presented. The rolling speeds that provide a liquid friction mode are given. The obtained relations allow us to estimate the influence of contact friction on technological parameters of sheet rolling and can be used in technological calculations of this process

Effects of multiple flexible vortex generators on the hydrothermal characteristics of a rectangular channel

Plenty of numerical or experimental studies have been devoted to the hydrothermal characteristics of heat sinks with multiple vortex generators (VGs). In this study, in order to boost the heat transfer rate and reduce the hydrodynamic dissipations, VGs are chosen to have flexible splitter on their lee sides. Also, VGs are selected at different axial and transverse inter-distances. The Reynolds number, based on the channel's hydraulic diameter, is set to 1000 to ensure laminar flow. The heat transfer performance, the hydrodynamic friction factor and the overall hydrothermal efficiency for different number of VGs and splitter's flexural rigidity are investigated. The results showed that flexible splitters provide better heat transfer capability and the higher hydrothermal performance compared to rigid ones. The results also indicated that, by selecting proper configuration, in the expense of 10% decrease in total hydrothermal efficiency with respect to clean channel, 190% increase in the rejected heat can be achieved.

Understanding the Mechanism of Load-Carrying Capacity between Parallel Rough Surfaces through a Deterministic Mixed Lubrication Model

Experimental results have confirmed that parallel rough surfaces can be separated by a full fluid film. However, such a lift-off effect is not expected by the traditional Reynolds theory. This paper proposes a deterministic mixed lubrication model to understand the mechanism of the lift-off effect. The proposed model considered the interaction between asperities and the micro-elastohydrodynamic lubrication (micro-EHL) at asperities within parallel rough surfaces for the first time. The proposed model is verified by predicting the measured Stribeck curve taken from literature and experiments conducted in this work. The simulation results highlight that the micro-EHL effect at the asperity scale is critical in building load-carrying capacity between parallel rough surfaces. Finally, the drawbacks of the proposed model are addressed and the directions of future research are pointed out.

Критическое скольжение в подшипниках качения при гидродинамическом трении

Критическое скольжение в подшипниках качения при гидродинамическом трении

Вплив трибологічної ефективності ресурсовизначальних спряжень деталей трансмісії на експлуатаційну надійність транспортних машин

This work examines the conditions of tribological efficiency of couplings of TM transmission discs as resource-determining parts and its influence on the operational reliability of machines. The change in hydrodynamic friction, oil consumption, oil carrying capacity by rotating disks in both laminar and turbulent modes of transmission oil flow is substantiated. The nature of oil discharge from the working surface of the disk in laminar and turbulent modes is considered. Based on the system of the Navier-Stokes equation, the change in the effective characteristics of the coupled transmission discs is substantiated: carrying capacity and oil consumption. The nature of oil discharge from the working surface of the transmission disc is considered, taking into account the friction forces and inertia forces. The dependence of the moment of hydrodynamic resistance on a number of parameters, as well as the relative moment of resistance on the Reynolds and Froude criteria, was investigated as the efficiency of the coupled discs of the transmission of transport vehicles. A qualitative graphical dependence of the moment of hydrodynamic resistance of tribocouplers of disks on the angular speed of their rotation is constructed, with the selection of characteristic zones, and a description of the physical picture of the processes in them is given. The well-founded dependence of the moment of hydrodynamic resistance on a number of influencing parameters, as well as the relative moment of resistance in the multiplicative model representation through criterion dimensionless complexes. The set of processes occurring in the tribo-coupling of disks depending on the angular speed of their rotation in five selected zones has been clarified. It is shown that the efficiency and reliability of the TM transmission depends on the total frictional force acting on the tribo-coupling of the discs, load and operating conditions.

Rotational relaxation dynamics study of rhodamine 800 in series of primary alcohols solvents

Rotational reorientation times of rhodamine 800 laser dye have been studied in a series of primary alcohols at room temperature by using steady-state and picosecond time-resolved fluorescence spectroscopy. Experimental rotational reorientation time shows, it increases linearly when the macroscopic viscosity of solvents increases. Hydrodynamic (SED) and Quasi hydrodynamic (GW and DKS) friction models qualitative analysis has been performed to determine mechanical friction contribution to the total friction experienced by laser dye. The results found interestingly that theoretically predicted rotational reorientation times underestimate experimental values.Deviations from the above models enable us to test for dielectric friction (NZ, ZH, and AW) theories and hydrogen bonding possibility between free clo-4 radical and –OH group of alcohol.

Повышение износостойкости радиального подшипника c нестандартным опорным профилем и полимерным покрытием на поверхности вала с учетом зависимости вязкости от давления

The paper covers the development and analysis of a model of the true viscous lubricant movement in the working gap of a radial sliding bearing with a non-standard support profile having a fluoroplastic composite polymer coating with a groove on the shaft surface. The authors obtained new models based on the classical equation in the approximation for the “thin layer” and the continuity equation describing the laminar pattern of movement of a lubricant with the viscous rheological properties. The results of the numerical analysis of the developed models allowed obtaining a quantitative assessment of the efficiency of the bearing bush support profile and the polymer-coated shaft with an axial groove. To complete the set of studies and verify theoretical insights, the authors carried out the experimental research. The novelty of the work lies in the development of an engineering calculation technique that allows determining the magnitude of the main tribotechnical parameters of a radial sliding bearing (hydrodynamic pressure, load capacity, and friction ratio) and expanding the area of practical application of the developed engineering calculations. The design of the radial bearing with a fluoroplastic antifriction composite polymer coating, a 3 mm wide groove, and a special support profile ensured the stable shaft ascent on the hydrodynamic wedge, which experimentally confirmed the correctness of the results of theoretical studies of sliding bearings with a diameter of 40 mm at a sliding speed of 0.3–3 m/s and a load of 13–65 MPa.

Roles of viscosity, applied load and surface wettability on the lubrication behaviour of model liquid/semi-solid foods: Measurements with a bespoke tribo-cell fixture and rotational rheometer

Thin film sliding and friction phenomena of food bolus confined between tongue-palate surfaces during oral processing can be explored using tribological measurements. However, these measurements are still limited within the food industry due to the requirement of expensive commercial instruments which are not commonly used in the food industry. This work has designed and manufactured a modular “tribological cell” (tribo-cell) that can simulate lubricated soft-hard contact interfaces and can be mounted on a rotational rheometer to perform tribological measurements. The tribo-cell was validated by performing tribological measurements using a range of corn syrup solutions as model liquid foods. It was shown that the Stribeck curve describing the change in friction behaviour with entrainment speed or with the product of entrainment speed and liquid viscosity could be obtained. Since tribology deals with surface property, the cell was then used in the further studies to demonstrate the effects of applied normal load and surface wetting on the tribological response of lubricated hard-soft contact of the designed fixture. These parameters were shown to have a marked influence on in the boundary and mixed-lubrication regimes. The designed tribo-cell was also used to illustrate the impact of fat content on the lubrication properties of commercial liquid and semi-solid foods with different fat contents, thus, pointing out to the importance of tribology as a vital tool for product formulation designs in food and beverage industry.

Effects of a Cylinder Liner Microstructure on Lubrication Condition of a Twin-Land Oil Control Ring and a Piston Skirt of an Internal Combustion Engine

It is hypothesized that structured sliding-surfaces improve the lubrication condition by forming an oil sump on the sliding surface, redistributing the oil, and trapping wear debris. For these reasons, structured sliding-surfaces have been used as a friction reduction method for a long time. In this work, effects of microstructure laid on the cylinder liner of an internal combustion engine on twin-land oil control ring (TLOCR) and piston skirt lubrication condition were investigated by comparing friction between a conventional fine-honed liner (CFL) and a micro-structured liner with the CFL. The measurements using a floating liner engine showed that the microstructure improved lubrication condition by reducing hydrodynamic friction. On the other hand, it was also observed that the microstructure could result in elevated friction under certain engine operating conditions.Graphical Abstract

Lubrication and Sensory Properties of Emulsion Systems and Effects of Droplet Size Distribution.

The functional and sensory properties of food emulsion are thought to be complicated and influenced by many factors, such as the emulsifier, oil/fat mass fraction, and size of oil/fat droplets. In addition, the perceived texture of food emulsion during oral processing is mainly dominated by its rheological and tribological responses. This study investigated the effect of droplet size distribution as well as the content of oil droplets on the lubrication and sensory properties of o/w emulsion systems. Friction curves for reconstituted milk samples (composition: skimmed milk and milk cream) and Casein sodium salt (hereinafter referred to as CSS) stabilized model emulsions (olive oil as oil phase) were obtained using a soft texture analyzer tribometer with a three ball-on-disc setup combined with a soft surfaces (PDMS) tribology system. Sensory discrimination was conducted by 22 participants using an intensity scoring method. Stribeck curve analyses showed that, for reconstituted milk samples with similar rheological properties, increasing the volume fraction of oil/fat droplets in the size range of 1–10 µm will significantly enhance lubrication, while for CSS-stabilized emulsions, the size effect of oil/fat droplets reduced to around 1 µm. Surprisingly, once the size of oil/fat droplets of both systems reached nano size (d90 = 0.3 µm), increasing the oil/fat content gave no further enhancement, and the friction coefficient showed no significant difference (p > 0.05). Results from sensory analysis show that consumers are capable of discriminating emulsions, which vary in oil/fat droplet size and in oil/fat content (p < 0.01). However, it appeared that the discrimination capability of the panelist was significantly reduced for emulsions containing nano-sized droplets.