Hydrodynamic Friction Research Articles

On the experimental characterization of the fluid volume influence on the friction between rough surfaces

The load-bearing behaviour of lubricated contacts depends primarily on the normal force, the relative velocity, and the geometry. Thus, with the aid of the Stribeck curve, it is usually well possible to characterize whether hydrodynamics, mixed friction, or boundary friction is more likely to be present. The fact that the load regime can also depend on the fluid quantity is obvious, but has hardly been systematically investigated so far. Especially for contacts with microscopic roughness, the defined application of a very small amount of fluid is a very challenging requirement. In this paper, a very fundamental study shows how a pin-on-disc tribometer can be used to achieve the transition from dry friction via mixed friction to predominant hydrodynamics by the amount of supplied fluid. The experiments are carried out on samples filed with different coarseness. In addition, the simultaneous influence of partial filling and normal force as well as relative velocity is also shown. Very good reproducibility has been practically reached over the entire range of the tests. Regarding the quantities for the coefficient of friction (COF), it was concluded that close to full filling, a reduction of the fluid quantity has a similar effect on the COF as the reduction of the velocity. This property goes along with the common theory of starved lubricated systems. Such behaviour was not observed to the same extent for the normal force. In the vicinity of smaller fluid quantities, the COF increases very rapidly with further reduction in fluid quantity, far more disproportionately than that with reduction in velocity. With a deeper understanding of this problem, various practical issues such as idling or the run-up process in bearings can also be studied in a more focused manner.

Dynamic Friction Model for the Investigation of Stick‐Slip Oscillations

AbstractThe optimization of lubricated sealing systems with respect to the stick‐slip effect requires a friction model that describes the complex friction behavior in the lubricated contact area. This paper presents an efficient dynamic friction model based on the Stribeck curve, which allows to investigate the influencing parameters through finite element (FE) simulations. The simulation of a tribometer test using this friction model proofs that the model correlates well with the tribometer test results. It is shown that the system stiffness has a significant influence on the stick‐slip tendency of the system.

Study on hydrodynamic lubrication and friction reduction performance of spur gear with groove texture

Spur gears are widely used in aerospace, energy power and other fields due to their advantages of convenient design and manufacturing. However, in the process of spur gear transmission, the tooth surface wear and adhesion are prone to failure, which seriously affect the life of the gear transmission. In order to minimize the frictional wear during gear transmission, a groove-type tooth surface texture is proposed in this paper. The core size parameters of the groove texture are optimized through CFD (Computational Fluid Dynamics) fluid simulation and groove texture cylindrical roller process experiments. Gears with groove texture are prepared by using the laser processing technology, and comparative friction wear tests are conducted. The experimental results show that the maximum and minimum damage area rates of the spur gears with groove texture are, respectively, 84.8% and 84.9% lower than those of the spur gears without groove texture. The anti-adhesion ability is also significantly improved in the gears with grove texture. In addition, the average friction coefficient of the spur gears with groove texture is significantly reduced, which effectively reduces the transmission dynamic pressure lubrication and friction in the spur gears.

Dynamics of Roller Bearings of Gas Turbine Engines under Hydrodynamic Friction

Dynamics of Roller Bearings of Gas Turbine Engines under Hydrodynamic Friction

Extrusion and rarefaction of lubricant in boundary layer is the key processes of adhesive wear of highly loaded tribocontacts

A comprehensive analysis of the Adhesion-Deformation, Elasto-hydrodynamic and Hydrodynamic friction models is presented, which describe different modes of lubrication in accordance with the Stribeck curve. The main provisions of these models are considered in conjunction with the Langmuir-BET theory of adsorption and Hertz's elastic-deformation theory of curvilinear contacts. It is shown that the revealed contradictions require their resolution, and the discovered multiple effects need a scientifically based interpretation. It is proposed to develop a more generalized model of friction and wear based on naturally occurring processes that have been hidden from direct observations for a long time. These are: Extrusion of lubricating layers in the convergent elastically deformed and Rarefaction in divergent elastically deformed regions of tribo-contacts. Understanding these processes makes it possible to predict the localization sites and causes of the occurrence of primary subsequent acts of adhesion of friction surfaces and their wear in the following cycle: “rarefaction and desorption of lubricating layers, which leads to deformation destruction of oxide films and adhesion of juvenile surface areas, after which to tearing of a fragment material from the bearing and the neoplasm of the protrusion on the shaft - in the divergent elastically deformed areas of the contact. Then microcutting by this fragment of the bearing surface occurs with the release of the wear product in the convergent elastically deformed region, which accordingly leads to a change in the actual geometry and tension of the tribo-contact. Further, in other areas of the renewed contact, adhesive interaction occurs in other divergent areas according to the same mechanism. A deep understanding of the reasons for the desorption of lubricating layers will make it possible to develop and apply new highly efficient technological and material science methods in order to increase the resource of highly loaded tribo-systems of machines and mechanisms.

Line contact lubricated by a fluid described by non-Newtonian Giesekus model

Abstract Modelling hydrodynamic lubrication helps engineers and material scientists designing lubricants that minimize viscous energy losses without compromising the durability of engine components. This type of modelling requires selecting lubricant rheological constitutive equations that provide realistic information on fluid behaviour within tribological contact zones. Due to complexity of differential constitutive equations the modelling effort was limited to generalized Newtonian or Oldroyd-B fluids. In this paper, we present more realistic description of hydrodynamic lubrication using the Giesekus fluid capturing both viscous shear thinning and the development of elastic stresses at high Weissenberg numbers. The problem analysis is done using a modified regular perturbation method with respect to the small mobility parameter $\alpha $. The perturbation analysis modification is caused by the fact that the problem contains several a priori unknown boundaries. The actual perturbation solutions for functions of pressure and gap, as well as the coordinate of exit from the contact and the exit film thickness, are obtained as linear functions of $\alpha $. The effects of the model parameters such as the mobility factor, relaxation time and the polymer contribution to viscosity on pressure, shear and extensional viscosities within the contact zone are analysed. It is shown that the presence of polymers helps to increase the gap and reduce the viscous losses associated with hydrodynamic friction. This result is consistent with empirical observations of fuel economy enhancement for polymer-containing lubricants.

Designing a Simple Test to Measure and Rate Lubricant-Controlled Friction Reduction

The ability of a lubricant to reduce friction and increase machine efficiency is one of its main functions. However, a standardized test to measure the ability of a lubricant to reduce friction does not exist. In this work we take a common tribometer and design a test sequence to allow the friction reduction of any lubricant to be measured and rated, quickly and accurately. The kinematic viscosity values at 40 and 100 °C for the lubricant, along with the density, are used to calculate appropriate entrainment speeds to maintain a constant test severity for each lubricant. A set sliding distance is determined experimentally to ensure that the metal surfaces are run in and that a stable tribofilm is formed at both temperatures. The lubricants are then rated in terms of their ability to maintain low friction in the boundary, mixed, and full film regimes. The proposed method has been designed ensuring good repeatability, across a range of common lubricants (engine oils, transmission fluids, hydraulic fluids, gear oils). This new method simplifies friction measurement and rating of lubricants using Stribeck curves, supporting the development and qualification of new lubricants.

Temperature-dependent wall slip of Newtonian lubricants

We discuss (i) the measurement, (ii) interpretation and (iii) technical impact of temperature-dependent wall slip and its activation energy for polar and non-polar hydrocarbon Newtonian fluids moving relative to machined metal surfaces. A newly developed apparatus, the slip length tribometer (SLT), overcomes the drawbacks of existing measurement devices in terms of characterising relevant rough surfaces by measuring the slip length at different temperatures over a sufficiently large wetted area. The experimental data show that the bulk viscosity and slip length at the fluid–metal interface is independent of the shear rate up to $10^5\,\mathrm {s}^{-1}$ , being consistent with recent results from molecular dynamics simulations by Mehrnia & Pelz (J. Mol. Liq., vol. 336, 2021, 116589). Furthermore, the activation energies for wall slip and bulk shear determined by means of the SLT differ by a factor of two, i.e. $E_{\lambda }\approx 0.5 E_\mu$ for non-polar hydrocarbon molecules sliding relative to metal walls. This difference is explained by a generalised Eyring model applied to wall slip. The paper closes with the impact of wall slip on Sommerfeld's similarity theory of tribology and the resulting Stribeck curve (Sommerfeld, Z. Math. Phys., vol. 50, 1904, pp. 97–155; Z. Tech. Phys., vol. 2. Jahrg., 1921; Mechanik der deformierbaren Medien, 1944, Akademische Verlagsgesellschaft Becker & Erler). For this purpose, Reynolds’ equation generalised for wall slip is solved in combination with the experimentally determined constitutive relations for bulk shear and wall slip to predict typical characteristics of journal bearings. The results show that, for a typical journal bearing, where the ratio of slip length to average bearing clearance is of the order of $10^{-2}$ , the influence of wall slip on both load-carrying capacity and dissipation is not negligible. This work combines nanofluidics and tribology in order to provide methods and knowledge for e.g. tailor-made fluids and interfaces.

Instrumental characterization of xanthan gum and scleroglucan solutions: Comparison of rotational rheometry, capillary breakup extensional rheometry and soft-contact tribology

Xanthan gum and scleroglucan, two rod-like polysaccharide hydrocolloids, are compared using a wide range of instrumental techniques and methods: steady shear flow, small amplitude oscillatory shear, first normal stress difference, capillary break-up and soft-contact tribology. The aqueous solutions of these two hydrocolloids with similar flow and viscoelastic profile show marked differences in capillary break-up time and apparent extensional viscosity. This result correlates with differences in first normal stress difference and, to a lesser extent, Stribeck curve behaviour. Formulating the hydrocolloids in concentrated sucrose solution (40 wt%) shifts relaxation profiles to longer times which greatly diminishes differences in rheological and lubrication behaviour. With exception of capillary break-up tests, no other methods showed statistically significant differences between the polysaccharides dissolved in the viscosified matrix. The toolbox of techniques is also applied to probe interactions of xanthan gum and scleroglucan with human whole saliva and bovine submaxillary mucin. We report no specific interactions between either hydrocolloid and salivary proteins and suggest that any cumulative effects must stem from specific sets of linear and non-linear rheological properties of saliva/hydrocolloid mixtures. • Rheology and tribology of xanthan gum and scleroglucan are compared. • Capillary break-up provides sensitive discrimination between hydrocolloids. • Xanthan gum and scleroglucan have similar hydrodynamic volume. • Relaxation spectrum of xanthan features faster modes compared to scleroglucan.

Engineering a ceramic piston pump to minimize particle formation for a therapeutic immunoglobulin: A combined factorial and modeling approach

AbstractDuring fill‐finish manufacturing, protein‐pump surface interactions can induce subvisible particle (SVP) formation which poses a risk to drug product quality and patient safety. Despite this risk, there have been no concerted efforts to understand the effects of piston pump design on SVP formation. We've systematically varied the design of the piston‐cylinder interface to minimize SVP formation for a therapeutic immunoglobulin. The clearance factor, surface roughness factor, and their combined interaction significantly affected particle concentrations, quantitated by light obscuration and microflow imaging. Optimized pump designs reduced particle levels by 1–2 orders of magnitude compared to the off‐the‐shelf equipment. At the piston surface, scanning electron microscopy revealed evidence of protein film abrasion, a process which ejects SVPs from the piston‐cylinder interface as wear debris. Computational fluid dynamics and quartz crystal microbalance were applied to simulate fluid flow and protein adsorption phenomena in the pump respectively. The risk of protein film abrasion was modeled along a hypothetical Stribeck curve, thereby interconnecting design parameters, lubrication conditions, and SVP formation. Our findings support implementation of a modular pump platform with interchangeable pistons; this approach would enable the pump design to be customized based on each protein's propensity to form SVPs. This flexible approach can benefit pharmaceutical manufacturers and patients alike by accelerating tech transfer and improving process control.

Critical Sliding in Rolling Bearings under Hydrodynamic Friction Conditions

Critical Sliding in Rolling Bearings under Hydrodynamic Friction Conditions

Tribological Properties of Ti6Al4V Titanium Textured Surfaces Created by Laser: Effect of Dimple Density

The loss of energy due to friction is one of the major problems industries are facing nowadays. Friction and wear between sliding components reduce the mechanical efficiency of machines and have a negative impact on the environment. In recent years, surface texturing has shown tremendous ability to reduce friction and wear. Micro-features generated on surfaces act as a secondary reservoir for lubricants and wear debris receptacles to further reduce abrasion. In addition, surface texturing boosts hydrodynamic pressure, which increases the elasto-hydrodynamic lubrication regime of the Stribeck curve, reducing friction and wear. Amongst all different techniques to texture surfaces, laser texturing is the most popular due to its advantages such as high accuracy, good consistency and celerity as compared to other techniques. This study investigated the effect of laser texturing on the tribological properties of Ti6Al4V in contact with a ceramic ball. The effect of varying the dimple density on friction and wear was studied using a ball-on-flat reciprocating tribometer under lubricated conditions. Results show that friction and wear were reduced for all the textured samples as compared to an untextured sample, with important friction and wear reductions for the samples with the highest dimple densities. For samples with intermediate dimple densities, the friction coefficient stayed low until the dimples wore out from the surface and then increased to a value similar to the friction coefficient of the untextured surface. The dimple wear-out time observed in these specimens was greatly influenced by the dimple density.

Coefficient of friction behavior of gear oils and significance for the meshing process of spur gears

The mechanical properties of oils are determined using test methods. There are standardized test methods for determining viscosity and density. The characterization of transmission oil based on its dynamic viscosity alone is not sufficient for the physical explanation of different levels of noise emissions in vehicle transmissions. For this reason, the test procedure for determining the coefficient of friction is used in the following to enable a further differentiation between the oils according to mechanical properties.In gear transmissions with involute gear teeth, rolling friction occurs in the gear pair meshing along the line of action due to the variation in the equivalent curvature radii throughout the meshing cycle. This is rolling friction on which a sliding friction component, so-called slip, is superimposed. Pure rolling friction only occurs in the pitch point. From the pitch point to the start and end of the meshing, there is a superimposed sliding friction component that increases with increasing distance from the pitch point. Slip values occur in the range of 5–50% depending on tooth geometry.These friction conditions during tooth flank lubrication can be assessed using the Stribeck curve. The Stribeck curve represents the coefficient of friction as a function of the speed. A mini traction machine from PCS Instruments with a ball/plate measurement setup was used to determine the coefficient of friction behavior of gear oils. This allows the coefficient of friction of an oil to be assessed at low speeds in the range from boundary and mixed friction to elastohydrodynamic fluid friction at high speeds.The investigations show that the coefficient of friction behavior of a gear oil depends on the oil viscosity and above all on the chemical composition. The lower the coefficient of friction, the less energy is required to shear the lubricating film and the lower the power transmission through the fluid. The coefficient of friction is a property that is dominated by the type of base oil and the type of VI improver in the area of mixed and fluid friction, especially with additional sliding in contact.It will be shown in the paper that the use of a gear oil that has been optimized with regard to the coefficient of friction curve can reduce the entry impacts of meshing gear pairs under vibration excitation and the gear transmission thus generates lower noise emissions.

Methyltrioctylammonium Octadecanoate as Lubricant Additive to Different Base Oils

This study investigates the use of an ionic liquid obtained from fatty acids (FAIL) as an additive at 2 wt.% in two different base oils: a mineral oil (M1) and a polyol ester (E1). Physicochemical characterization of the base oil–FAIL blends confirmed the miscibility of the FAIL in the base oils. The addition of the FAIL hardly changed the density of the base oils and the viscosity slightly increased at lower temperatures. The tribological performance of the base oils and their blends with the FAIL was determined using three different tests: Stribeck curve determination and tribofilm formation tests, both under sliding/rolling motion, and reciprocating wear tests. The M1 + FAIL blend showed the lowest friction values under the mixed lubrication regime due to its higher viscosity, while the E1 + FAIL showed the lowest friction values under the elastohydrodynamic lubrication regime, which may well have been due to its higher polarity. Only the E1 + FAIL blend outperformed the antiwear behavior of the base oil, probably because it has better chemical affinity (higher polarity) for the metallic surface. SEM images showed that the predominant wear mechanism was adhesive-type with plastic deformation and XPS studies proved that the presence of increasing amounts of organic oxygen on the wear scar caused better antiwear performance when the E1 + FAIL blend was used.

Nanomechanical tribological characterisation of nanostructured titanium alloy surfaces using AFM: A friction vs velocity study

Medical-grade titanium alloys used for orthopaedic implants are at risk from infections and complications such as wear and tear. We have recently shown that hydrothermally etched (HTE) nanostructures (NS) formed on the Ti6AlV4 alloy surfaces impart enhanced anti-bacterial activity which results in inhibited formation of bacterial biofilm. Although these titanium alloy nanostructures may resist bacterial colonisation, their frictional properties are yet to be understood. Orthopaedic devices are encapsulated by bone and muscle tissue. Contact friction between orthopaedic implant surfaces and these host tissues may trigger inflammation, osteolysis and wear. To address these challenges, we performed simulation of the contact behaviour between a smooth control Ti6Al4V alloy and HTE surfaces against a hardwearing SiO2 sphere using Atomic Force Microscopy (AFM) in Lateral Force Microscopy mode. The friction study was evaluated in both air and liquid environments at high (5 Hz) and low (0.5 Hz) scan velocities. Lower scan velocities demonstrated opposing friction force changes between the two mediums, with friction stabilising at higher velocities. The friction measured on the NS alloy surfaces was reduced by ~20% in air and ~80% in phosphate buffered saline, in comparison to the smooth control surface, displaying a non-linear behaviour of the force applied by the AFM tip. Changes in friction values and cantilever scan velocities on different substrates are discussed with respect to the Stribeck curve. Reduced friction on nanostructured surfaces may improve wear resistance and aid osseointegration.

The study of the vibration under different lubrication states based on Stribeck curve

The tribology system is a complex nonlinear dynamical system. To study the vibration under different lubrication states, the experiments on full-film lubrication state and mixed lubrication state are designed based on Stribeck curve. In full-film lubrication state, vibration signal energy does not increase, but in mixed lubrication state, vibration signal energy increases with the increase of loading force. Under the condition of full-film lubrication, the autocorrelation function of vibration signal is similar to that of noise signal, which has smooth characteristics. In mixed lubrication state, the autocorrelation function of vibration signals has the characteristics of narrow convergence. Results reflect that for smaller loads there are no friction-induced vibrations due to full-film lubrication and for higher loads the friction-induced vibration comes into existence as the rough peaks between pin and disk surfaces come into play.

Adaptive force tracking control of electrohydraulic systems with low load using the modified LuGre friction model

The force tracking performance of controllers is seriously degraded by the complex friction behaviors when the electrohydraulic system applies low load. The existing LuGre model is employed in the motion control system with the known trajectory. In this work, a modified LuGre model combined with a zero-velocity crossing window is established to address the lag issue and model oscillation in force control system. Then, the Stribeck curve in the friction model is accurately obtained using a fast measurement method, and the impact of friction parameters on the accuracy of the LuGre model is analyzed. Next, an adaptive friction compensation control scheme is proposed for the electrohydraulic system with low load using the modified LuGre model to compensate for dynamic friction behavior and improve the force tracking performance. The uncertain friction parameters are tuned online to reduce the force error of the low-velocity region via the derived adaptive law and a dual-observer. The disturbance observer is effectively integrated to reduce the negative influence of unconsidered dynamics. Finally, comparative experiments are conducted to verify the effectiveness of the proposed controller in static and dynamic loading modes.

Soft Tribology and Its Relationship With the Sensory Perception in Dairy Products: A Review.

Nowadays, dairy products, especially fermented products such as yogurt, fromage frais, sour cream and custard, are among the most studied foods through tribological analysis due to their semi-solid appearance and close relationship with attributes like smoothness, creaminess and astringency. In tribology, dairy products are used to provide information about the friction coefficient (CoF) generated between tongue, palate, and teeth through the construction of a Stribeck curve. This provides important information about the relationship between friction, food composition, and sensory attributes and can be influenced by many factors, such as the type of surface, tribometer, and whether saliva interaction is contemplated. This work will review the most recent and relevant information on tribological studies, challenges, opportunity areas, saliva interactions with dairy proteins, and their relation to dairy product sensory.

A method for the tribological assessment of oral pharmaceutical liquids

Objective Patient acceptance of pediatric formulations is critical to compliance and consequently therapeutic outcomes; thus, having an in vitro method to evaluate sensory perception of pharmaceutical products would be beneficial. The objective of this research is to develop a sensitive and reproducible tribological method to characterize pharmaceutical suspensions at low force and sliding speeds. Methods The discriminating potential of the method was examined using tribology profiles (coefficient of friction (COF) vs. sliding speed) for commercially available products and products made for this study with widely varying sweetness, thickness, and grittiness; these formulations were used to judge the sensitivity of the method. Samples were measured using 3M Transpore™ surgical tape to simulate the tongue surface, steel half ring geometry, constant gap setting, target axial force of 2 N in a 600 s exponential ramp for rotation speed. Results The COF ranged from 0.1 to 0.6. For the speeds studied, the high viscosity commercial suspension ibuprofen drops and acetaminophen suspension show a classic Stribeck curve with an increasing COF at the higher rotation speeds, which indicates these formulations entered the hydrodynamic lubrication phase, while the lower viscosity suspensions only reached the mixed lubrication phase. Conclusion The contribution of particles affects the COF in a dynamic tribologic pattern compared to products that are categorized as either low gritty or high viscosity. These results are important as they provide a potentially rapid in vitro method for screening pediatric medications and help to identify the factors that affect the palatability of pediatric formulations.

Tribological properties of hierarchical micro-dimples produced on a cylindrical surface by dual-frequency texturing

An experimental investigation was performed for investigating the tribological performance of micro-dimple surface texture patterns on a cylindrical surface in a realistic operating environment of starved lubrication. Micro-dimples were generated by a dual-frequency surface texturing method, in which a high-frequency (16.3 kHz) three-dimensional (3D) vibration and a low-frequency (230 Hz) one-dimensional (1D) vibration were applied at the tool tip simultaneously, resulting in the generation of the hierarchical micro-dimples in a single step. Rotating cylinder-on-pin tribological tests were conducted to compare the tribological performance of the non-textured reference specimen and micro-dimple samples. The effect of surface textures generated with various shape parameters (long drop and short drop), dimension parameters (length and surface texture density), and operation parameters (load and sliding velocity) on the tribological performance was evaluated. Stribeck curves indicate that the hierarchical micro-dimples exhibit a lower coefficient of friction than the reference specimen in the high contact-pressure regions. It is also observed that variation in the length of a micro-dimple, the shape effect, is the major factor affecting the friction response of the textured surfaces. The generation of additional hydrodynamic pressure and lift effect by hierarchical structures is the main reason for the improved performance of hierarchical micro-dimple surfaces.