Lubricant Film Thickness Research Articles

Lubrication Film Generation with Limited Lubricant Supply and Its Tuned Oil Replenishment in a Cylinder-on-Disc Contact

Limited lubricant supply (LLS) with tuned dosage is an emerging approach to reduce friction and energy consumption. However, LLS can cause severe starvation when the oil supply is insufficient. Therefore, how to effectively replenish oil to the lubricated contact in LLS operation is very important. Using a custom-made optical slider bearing test rig, this work experimentally revealed some characteristics of LLS lubrication in a cylinder-on-disc contact, and proposed two wettability gradient patterns, namely stripe wettability and interlaced wettability, to regulate the lubricant supply to improve the lubricating properties of LLS. The effect of these two wettability patterns was evaluated experimentally according to the lubricating film thickness. The interferograms of the bearing contact under LLS show that the two patterns can augment the oil replenishment through unidirectional lubricant transport by the unbalanced interfacial force via the wettability gradient. Moreover, the interlaced wettability pattern is more effective due to the discontinuous distribution of lubricant from its hydrophilic/hydrophobic region intervals and the transfer of the hydrophobic film to the hydrophilic regions.

A unified model for large-scale thickness measurement of lubricating film based on ultrasonic lag phase slope

A unified model based on the ultrasonic lag phase slope is developed for measuring the lubricating film thickness at a large scale. The ultrasonic lag phase of adjacent waves instead of the phase of overlapped waves is calculated as a function of the ultrasonic frequency and film thickness. The slope of the ultrasonic lag phase is determined correspondingly, which is linearly proportional to the lubricating film thickness. Both the finite element analysis and tests on the lubricating film thickness are performed to verify the proposed method. The results show that despite the fluctuations of the lag phase, the lag phase slope can be used for measuring the lubricating film thickness at a large scale from 0.1 μm to 170 μm.

An improved method for film thickness measurement of cylindrical roller bearings based on a straight-beam probe

PurposeConsidering the industrial site environment and installation requirements, the straight-beam ultrasound probe with easy installation and good coupling agent adaptability is adopted to replace the traditional water immersion focusing probe for film thickness measurement in cylindrical roller bearings. The straight-beam probe has a large echo receiving range, which will result in measurement regions overlapping and bring about large measurement errors. In this paper, an improved measurement method for film thickness of cylindrical roller bearing with the straight-beam probe is developed.Design/methodology/approachAn improved method is proposed to enhance the spatial resolution of the straight-beam probe. By introducing a correction coefficient based on the percentage of the effective measurement area, the method improves the measuring accuracy successfully.FindingsThe experimental results demonstrate that the lubricant-film thickness can be measured to reasonable accuracy by this method and have a better agreement with the theoretical film thickness solutions.Originality/valueThis paper used analytical method and model that is helpful for the improvement of the spatial resolution, which has great influence on the measuring accuracy, is mainly determined by the echo reflection area size of the ultrasound transducer.

High-Speed Digital Photography of Vapor Cavitation in a Narrow Gap Flow

Digital photography of cavitation in narrow gap flows, e.g., lubrication films in journal bearings or squeeze film dampers, demands a high time-resolution and a solution to approaching the particular spatial restrictions. Typically, the lubrication film thickness is in the range of a few microns and the characteristic time for vapor bubble generation and collapse is about one millisecond, respectively. The authors have developed a Journal Bearing Model Experiment, which is designed according to similarity laws providing fully similar flow conditions to real journal flows while offering ideal access to the flow by means of optical measurement equipment. Compared with other methods, e.g., pulsed laser, electrical discharge, tube arrest, applied to produce vapor bubbles, the work on hand applies a dynamic variation of the minimum film thickness to produce suction cavitation, which proves the applicability of this novel approach to study vapor cavitation in fluid films similar to lubricant flows. The results are obtained by means of digital high-speed photography of vapor bubbles from inception to implosion triggered by the dynamic variation of the minimum film thickness of a narrow gap flow. Moreover, the results are set in relation to a general overview of cavitation processes.

Temperature Effect on Load Distribution, Friction, and Wear of a Grease-Lubricated Spherical Roller Bearing (SRB)

This article investigates the effect of temperature on the lubrication and wear behavior of a grease-lubricated spherical roller bearing (SRB) assuming mixed-elastohydrodynamic lubrication (mixed-EHL) where the contact load is shared between the lubricant film and contact asperities. Temperature effects on internal load distribution and elastic modulus and their subsequent effect on the wear are also analyzed. The effect of temperature is investigated, first on the film thickness and load sharing, and then on the friction and wear. If the temperature rises from ambient to 110 °C, the increase in the contact patch dimensions, due to the decrease of elastic modulus, is only approximately 1% and the decrease in mean contact pressure is approximately 2%. Moreover, the load at decreases by approximately 4%, and the load zone increases by approximately 5%. As the temperature rises from 50 to 110 °C, the asperity portion of the contact load at position is increased by 350%. At 50 °C, only 20% of the total traction is contributed by the contact asperity, which is 80% at 110 °C. For the temperature rise from 50 to 110 °C, the wear volume loss increases by 238% and nonlinearly, due to viscosity reduction and its effect on the reduction of film thickness. The isolated change of internal load distribution and elastic modulus due to temperature rise causes a relatively insignificant change in wear. Hence, the predominant effect of temperature rise on wear is due to a decrease in lubricant film thickness causing the asperity component of the contact load to increase.

Calculation and Lubrication Characteristics of Cylindrical Roller Bearing Oil Film with Consideration of Thermal Effects

Aiming at the problem of how the thermal characteristics of cylindrical roller bearings affect the lubrication characteristics of bearings under actual working conditions, the influence of parameters such as speed and load on the lubrication characteristics of cylindrical roller bearings under thermal effects is analyzed. The numerical calculation method combining the quasi-static model of cylindrical roller bearing and the thermal elastohydrodynamic lubrication model is adopted. The effects of rotational speed, load and thermal effect on the lubrication performance of the bearing and the lubrication state under certain oil supply conditions were analyzed via numerical model calculation. The oil film thickness was measured via an immersion ultrasonic method to verify the correctness of the model. The results show that the larger the bearing speed, the larger the central film thickness and the minimum film thickness. At the same time, the thermal effect on the film thickness is more obvious; the greater the load, the greater the maximum oil film pressure. The film thickness gradient in the inlet region is greatly reduced, but the thermal effect has no obvious effect on the overall film thickness. In addition, there is a critical value of effective lubrication film thickness for each set of operating parameters. When the actual film thickness is equal to the critical value, the bearing lubrication state is at its best; the numerical simulation results are compared with the experimental values. Under the calculation conditions, the maximum error at the measuring point is within 10%, which meets the error requirements and provides a theoretical basis for revealing the bearing lubrication mechanism.

Droplet transition from non-axisymmetric to axisymmetric shape: Dynamic role of lubrication film in a rectangular microfluidic channel

In the past 20 years, droplet microfluidics is burgeoning in many chemical and biological applications due to the unique capability of droplets to act as confined containers. Confinement is ensured even in the case of squeezed droplets within microchannels much smaller than droplet volumes due to the presence of a lubrication thin film that prevents contact between droplets and the channel walls. The thickness of the lubrication film depends on the dynamics of the entire microfluidic system, affecting the actual droplet's shape and velocity. Therefore, this film is extensively studied to obtain insight into the dynamics of flowing droplets, especially when confined in small channels. Circular cross section channels are the most studied for their axial symmetry, but practical applications present most likely non-axisymmetric channels, as a result of fabrication processes, such as soft lithographic rectangular channels. The latter showed unique transitional morphological behavior of droplets, which assumes an axisymmetric or non-axisymmetric shape during their flow inside a non-axisymmetric channel, depending on the lubrication film. This work gives a comprehensive experimental characterization of the dynamics of the lubrication film during the droplet shape transition. We settled on a novel approach based on the optical diffraction of a localized light beam provided by two-facing optical waveguides integrated with the microfluidics circuit. The technique allows for studying the dynamics of flowing droplets and their relationship with the lubrication film thickness. Additionally, this experimental system enables a precise definition of two regimes of lubrication film, and the critical capillary number at which the transition occurs.

Tribotronic control of an ionic boundary layer in operando extends the limits of lubrication

The effect of electric potential on the lubrication of a non-halogenated phosphonium orthoborate ionic liquid used as an additive in a biodegradable oil was studied. An in-house tribotronic system was built around an instrument designed to measure lubricant film thickness between a rolling steel ball and a rotating silica-coated glass disc. The application of an electric field between the steel ball and a set of customized counter-electrodes clearly induced changes in the thickness of the lubricant film: a marked decrease at negative potentials and an increase at positive potentials. Complementary neutron reflectivity studies demonstrated the intrinsic electroresponsivity of the adsorbate: this was performed on a gold-coated silicon block and made possible in the same lubricant system by deuterating the oil. The results indicate that the anions, acting as anchors for the adsorbed film on the steel surface, are instrumental in the formation of thick and robust lubricating ionic boundary films. The application of a high positive potential, outside the electrochemical window, resulted in an enormous boost to film thickness, implicating the formation of ionic multi-layers and demonstrating the plausibility of remote control of failing contacts in inaccessible machinery, such as offshore wind and wave power installations.

Characterising the effects of simultaneous water and gasoline dilution on lubricant performance

The introduction of hybrid electric vehicles presents new operating conditions which can increase water and fuel dilution of the engine oil. The effects of this type of simultaneous water and fuel dilution are poorly understood. Therefore, here we have characterised its effects on 1) boundary film performance 2) Lubricant rheology and film thickness. This is achieved by testing four samples: 0W‐20, 0W‐20+gasoline blend, 0W‐20+H20 water-in-oil emulsion and 0W‐20+H20+gasoline water-in-oil emulsions. A blended gasoline surrogate was used with a composition matching that found in real used engine oils.Gasoline dilution reduces viscosity under all conditions in a predictable fashion but has negligible chemical effects. Meanwhile, water affects only low-shear/low-pressure viscosity and is surface active and thus affects antiwear film formation by an apparent competition for the surface. This results in thinner and smoother anti-wear films giving both lower friction and higher wear. These individual behaviours are largely maintained even when water and gasoline dilution are present in the same blend, suggesting negligible interaction between the two components.

Boundary lubrication and super lubricity, textured surfaces

Over last couple of decades, various regimes of lubrication attracted lots of attention. Among these regimes of lubrication, the least understood and studied are the regimes or boundary lubrication and lubricity. Better understanding these regimes of lubrication is necessary due to the fact that many moving joints such as, for example, bearings and gears at least part of their operating time work in these regimes. It may occur due to slow motion of counter parts or their relative overload. This paper proposes a model for analysis of these regimes of lubrication. Specifically, the model takes into account roughness of one of the contact surfaces. It is shown that for sufficiently high applied load it can be expected that contact surfaces are completely separated by a lubricant layer in spite of the presence of surface asperities. The definitions of boundary lubrication regimes and super lubricity are proposed. A method based on the method of matched asymptotic expansions is applied for analyzing such cases and an approximate analytical solution of the problem away from the contact region boundary is found. In the narrow inlet and exit zones adjacent to contact boundaries equations describing the main solution terms are derived. For the case of super lubricity, a resemblance of a superposition between the lubrication and surface roughness effects is determined and the effect of surface roughness on the lubrication film thickness is considered. Specifically, certain roughness conditions are identified for which the lubrication film thickness can be higher than for smooth surfaces while in other cases it can be lower. For the super lubricity regimes, the equations for the main pressure asymptotic terms in the inlet and exit zones are reduced to the corresponding equations for the smooth (not rough) surfaces for which a series of numerical solutions is obtained earlier. A general analysis of the effect of texturing on lubrication conditions in case of super lubricity is conducted based on a simple geometric criterion. Several models of textured surfaces and their influence on the lubrication film thickness in case of super lubricity regimes are considered.

Plasto-elastohydrodynamic lubrication of heterogeneous materials in impact motion

The analysis of plastic evolution and material inhomogeneity is required for an accurate description of lubrication under impact loading. This work develops a semi-analytical model for heterogeneous materials in impact motion under plasto-elastohydrodynamic lubrication (PEHL) with limited inlet oil. A plasticity loop is proposed to obtain the accumulative plastic strain iteratively based on the analysis of the stress state. The inhomogeneous inclusion within the materials is homogenized with unknown eigenstrains according to the equivalent inclusion method. The surface displacements induced by the plastic strains and eigenstrains are introduced into the gap between the contact bodies to update the lubrication film thickness until the convergence is achieved. The consideration of plastic strains and eigenstrains makes the model more realistic for PEHL under impact loading. The responses including the pressure, film thickness, starvation parameter, residual deformation, temperature, and subsurface elastoplastic fields during the impact–rebound process are discussed. After the model validation against the reference data, the effects of material inhomogeneity and strain hardening on the lubrication response, as well as the typical features of elastic perfectly-plastic materials, are investigated to provide guidance for material reliability analysis.

Analysis of a Low-Loss Gear Geometry Using a Thermal Elastohydrodynamic Simulation including Mixed Lubrication

Low-loss gears are an interesting design approach for increasing the efficiency and thermal load limits of gearboxes. The loss-optimized gear geometry concentrates sliding around the pitch point, which results in low load-dependent gear power losses. In this study, a method for modeling transient EHL (elastohydrodynamically lubricated) contacts in gear mesh considering mixed lubrication and thermal effects is introduced and applied to analyze the tribological behavior of a low-loss gear geometry. Special focus is placed on local frictional losses to analyze the role of the thermal effects of the gear mesh. Although a thermal reduction in fluid friction is observed, the overall effect on total frictional losses of the low-loss gear geometry is evaluated to be very small. The edge geometry strongly influences the lubricant film thickness and frictional power losses.

The influence of Λ ratio and surface roughness on the initiation and progression of micropitting damage

Micropitting is a major failure mode in gears and rolling bearings. Despite its practical importance, there currently exist no universally accepted design guidelines for its prevention primarily due to the great number of influencing parameters. In the absence of an established criterion, the Λ-ratio (the ratio of lubricant film thickness to surface roughness) is often used as a simple way to assess the risk of micropitting. In this paper we present new data to establish and decouple the individual effects of two of the most important influencing parameters in micropitting: the surface roughness amplitude and Λ-ratio. The experiments are conducted on a triple disc contact fatigue rig and carefully designed to decouple the effect of roughness from that of the Λ-ratio. To isolate the influence of the Λ ratio, we use specimens with the same and very tightly controlled surface roughness and then vary the film thickness by blending different viscosities of the same PAO base oil while keeping all other influential parameters constant. To isolate the influence of surface roughness, we keep the Λ ratio the same using the same PAO oils with appropriate viscosities while changing the RMS roughness of the specimen and keeping other roughness parameters as constant as possible. All tests use case carburised 16MnCr5 gear steel specimens. Results show that higher Λ ratio at fixed roughness produces less micropitting as may intuitively be expected. More importantly, at a fixed Λ ratio the amount of micropitting damage was extremely sensitive to the actual surface roughness, with lower roughness both lengthening the micropitting incubation phase and decreasing the rate of material loss in the micropitting wear phase. This shows that Λ ratio on its own cannot be used to assess the risk of micropitting because it is not able to account for the effects of different roughness levels on micropitting, despite including roughness in its definition. Furthermore, in relative terms micropitting was more sensitive to surface roughness than to Λ. In addition, we show that there exists a sufficiently low surface roughness below which micropitting is unlikely to occur even at extremely low Λ ratios. The results presented here can help in design of gears and other mechanical components to mitigate against micropitting.

HACIA PRÓTESIS DE RODILLA MÁS DURADERAS: EL MODELO DE PHAN-THIEN YTANNER Y LA LUBRICACIÓN ELASTOHIDRODINÁMICA

The synovial fluid is usually assumed to be Newtonian despite its viscoelastic behavior. In this work a model based on the Reynolds equation and the Phan-Thien and Tanner rheological law (PTT) is presented. The model, of the ellipsoid-on-plane type, contemplates the curvature of the articular surfaces and assumes the tibial component, typically made of ultra-high molecular weight polyethylene (UHMWPE), is deformed according to the column model. Using commercial software COMSOL Multiphysics version 5.2, the governing equations of non-Newtonian lubrication flow were simultaneously solved to find the component of the total stress tensor for the PTT model, in the direction perpendicular to the flow, and the lubricating film thickness. From these results the friction forces and friction coefficients on the articular surfaces were calculated. Simulations were carried out for different operating conditions corresponding to elasto-hydrodynamic lubrication, the predominant lubrication mechanism in the simple support phase of the gait cycle. The results show the importance of an adequate model to obtain quantitative information, considering the fluid as Newtonian would lead to overestimations of the lubricant film thickness. The results suggest that, as long as thin film lubrication is the lubrication regime, the viscosupplementation and consequent increase in relaxation constantλwouldlead to lower coefficients of friction. However, for the lubrication regime to be thin film lubrication, the relaxation constant of the fluid should be less than 2.5×10−5s.

Evolution of oil film formation in the bearing radial bearing housings

Slip bearings are machine components that provide support for rotating parts, usually rotating shafts or axles, take on the forces that load the parts concerned and work under conditions of relative sliding of the shaft journal surface onto the bearing (bearing shell) surface, while the two surfaces are separated by a film of lubricant. For the correct operation of the slip bearings, without systematic wear of the Journal and bearing shell surfaces, they must be separated by a sufficiently thick lubricant film to completely exclude direct contact of the two surfaces in relative movement.

Influence of slip effect on viscous dissipation heat and lubrication characteristics of Gas Journal Bearing: A multiscale analysis

As operating under quite high bearing speed with quite narrow clearance, the slip effect generally inevitable exists and closely relates to the performance of Gas Journal Bearing (GJB). However, singly the conventional macroscopic method may not be capable of fully exploring the influence of slip effect and seize tiny details such as slip velocity and flow vortex. In this article, both the Lattice Boltzmann Method (LBM) and the Reynolds equation method are applied to analyze the distributions of lubricant viscous dissipation heat, velocity, slip velocity, pressure, journal position and film thickness versus different operating parameters at multiscale. It shows that the slip effect weakens the squeezing function on lubricant film, reduces the viscous dissipation heat peaks, consumes the maximum pressure peaks, and decreases the bearing capacity, which is more obvious at larger Knudsen number. At higher bearing speed, the high viscous dissipation heat and pressure zone rapidly magnifies, in contrast, under larger bearing load, the high viscous dissipation heat zone concentrates.

Effect of asymmetric material entrance on lubrication in cold rolling

An Integrated Digital Image Correlation (IDIC) method has been applied to the cold rolling process, the images clearly visualise the geometry of the roll bite and the inlet zone. A key, novel finding is that despite using coils and applying entry tension, the strip can enter asymmetrically in the rolling mill. Moreover, it is shown that asymmetric strip feeding results in an uneven lubricant film thickness between top and bottom strip surface. A method is detailed to feed the strip truly symmetrically in the mill, a procedure that is necessary for a meaningful experimental validation of lubricant film thickness models. For symmetric rolling processes the measured film thickness corresponds significantly better with the theory than for asymmetric processes.

Effect of grease grades on the tribological and vibrational behaviours of a radial ball bearing: Conventional vs. micro-grooved outer races

Comparative tribo-dynamic studies of radial ball bearings possessing micro-grooved and conventional outer races, employing three different grades of lithium soap-based greases (NLGI grade 000, 2 and 3), are presented in this paper. This research has been conducted to explore and understand the effectiveness of grease grades on the performance parameters in the presence of a circumferential micro-groove on the outer race as compared to conventional case. Indication of lubricating film thickness formation (in terms of capacitance), frictional torque, vibrations, and rise in temperature have been assessed for the conventional and micro-grooved outer race bearings at different operating parameters ( pH = 0.9 to 1.4 GPa; speed = 1.4 to 4.2 m/s). The micro-grooved bearings (MB) provided the indication of better lubricating film formation in terms of capacitance reduction in the range of 11–19% as compared to the conventional bearing (CB) cases. However, the reduction in frictional torque, vibration and rise in temperature magnitudes have been found in the range of 14–26%, 6–23%, and 10–24% respectively, as compared to the CB cases.

A Novel Set-Up for In Situ Measurement and Mapping of Lubricant Film Thickness in a Model Rolling Bearing Using Interferometry and Ratiometric Fluorescence Imaging

This paper describes a unique experimental set-up constructed for studies of lubricant behaviour in an operating rolling element bearing including in situ quantitative measurements of film thickness in and around the element-raceway contact. The set-up is based on a deep groove ball bearing in which the outer race is made of sapphire to allow full optical access to the zone in which the rolling elements are loaded against it. This allows direct imaging of lubricant films under both steady-state and transient conditions and at contact pressures and rotational speeds representative of those present in real rolling element bearings. Optical interferometry is used to measure thin EHL films inside the ball–raceway contacts while a specific laser induced fluorescence approach, referred to as ratiometric fluorescence, is implemented to observe the lubricant distribution and quantify its thickness ahead of the ball–raceway contact. Results are presented to validate the accuracy of the method and to investigate the influence of bulk lubricant viscosity and bearing speed on contact film thickness, inlet starvation and lubricant distribution around the ball–raceway contact. To the best of our knowledge, the work described here is the first to directly measure lubricant distribution and EHL film thickness in a ball–raceway contact in an operating radial rolling bearing.Graphical

Duncan Dowson. 31 August 1928—6 January 2020

After completion of his PhD studies, under Derman Christopherson at Leeds, Duncan Dowson spent most of his life researching in tribology and writing its history. Building on the early studies of Osborne Reynolds and later Russian scientists, he made outstanding contributions to the field of elastohydrodynamic lubrication. This is the science of lubrication in which account is also taken of the deformation of the bearing material. An important parameter is the ratio of the elastrohydrodynamic lubricant film thickness to the bounding surface roughness. This is coupled to considerations of surface deformation and wear. In engines, an important consideration is the use of piston sealing rings to contain the hot combustion gases. Similar lubricant film and wear analyses are applicable to artificial hip and knee joints. A fruitful collaboration developed with the Rheumatism Research Unit at Leeds. and a novel pendulum machine was developed to measure the friction in artificial joints. At that time, low friction in hip joints was achieved with stainless steel components, with high molecular weight polytetrafluoroethylene (PTFE) for the cup. Wear rates at the interfaces were excessive and alternatives continue to be sought, including metal-on-metal joints. This has led to extensive experimental studies of the wear in such joints by Duncan and his colleagues, revealing dependencies upon the metal, its surface roughness and any added lubricants. These experiments included the compatibility of different metals in metal-on-metal joints, their interconnected fluid film roughness and wear rate.