Lubricants, Volume 7, Issue 12 (December 2019) – 11 articles

Lubricants that are used in miniaturized moving mechanical components generally serve under severe conditions, such as high temperature, high speed, and high load. Although alkyldiphenylethers (ADEs) are used as base oils for high-temperature greases, their tribological properties remain unclear. This study investigated the influence of the alkyl chains on the tribological properties of ADEs. Longer and more attached alkyl chains decreased the friction coefficient of ADEs under both reciprocating and continuous sliding conditions. Wear was found to be independent of the alkyl chain under reciprocating sliding conditions due to abrasion being caused by debris that was not readily removed. ADEs showed good anti-wear properties when used as either a lubricant or an additive under continuous sliding conditions. Much smoother surfaces in the friction track were observed in comparison to poly-α-olefin. Fourier-Transform infrared spectroscopy analysis suggested that the ether groups were attracted to the worn steel surface, phenyl groups became consequently perpendicular, and the attached alkyl chains repelled other substances and prevented further wear of the surface. Moreover, ADEs showed a high adaptability with traditional additives. Full article

The review summarizes the literature data and the authors’ own research results on the application of liquid crystals in tribology. It has been shown that both thermotropic (calamitic, discotic, cholesteric) and lyotropic (surfactants, chromonics) mesogens as tribological additives are able to optimize the properties of lubricating compositions when introduced even at low concentrations to oils and greases. A wide possibility of varying the chemical structure of mesogens and studying the relationship between their structure and tribological properties can be used for the desired (programmed) change of the quality of tribotechnical processes. The synergism of the combined use of mesogenic esters of cholesterol and carbon nanostructures as additives in improving tribological properties has been established. The use of synthetic lubricants in biological systems still requires further research as the experimental results obtained on models of joint prostheses in vitro conditions are significantly worse than the results obtained in vivo. Considering the annual loss of billions of US dollars worldwide due to the low efficiency of friction processes in the industry and the resulting wear, liquid crystals and the systems based on them can be the most effective way to optimize these processes. The present review will be useful for researchers and industrialists. Full article

We report the synthesis and characterization of two different types of Ni-based laminated composites (Types I and II). In Type-I composites, layers of Ni and Ti₃SiC₂ (Ni–Ti₃SiC₂) were interleaved with Ni, whereas in Type-II composites, Ni–Ti₃SiC₂ layers were interleaved with Al and Ni. The laminate thickness and Ti₃SiC₂ content in the individual Ni–Ti₃SiC₂ layers were systematically varied in both the composites. Detailed SEM studies showed that Ti₃SiC₂ particulates are well distributed in the Ni-matrix with little or no interfacial reactions with interparticle porosity. However, there were interfacial reactions between Ni and Al in Type II composites. In general, Type I multilayered composites had higher ultimate compressive strength (UCS) in parallel orientation as compared to perpendicular orientation (layers are aligned parallel or perpendicular to the wear surface then it will be referred to as parallel or perpendicular orientation). Comparatively, in Type II composites, the UCS was greater in perpendicular orientation as compared to parallel due to the presence of Al layers as bonding layers. Both the composite designs showed triboactive behavior against alumina disks and sensitivity to laminate thickness and orientation. In Type-I composites, the decrease in µ and wear rate (WR) with laminate thickness was more pronounced in the perpendicular orientation as compared to the parallel orientation. Comparatively, Ni–Ti₃SiC₂/Al/Ni composites showed that the parallel orientation was more effective in enhancing the triboactive performance. SEM analysis of tribosurfaces showed signs of triboxidation and abrasion, which led to the formation of O-rich tribofilms. Full article

Rolling bearings are frequently used machine elements in mechanical assemblies to connect rotating parts. Resource efficiency and reliability enhancement are considered to be important factors of rolling bearing development. One of the ways to meet these requirements is the tailored forming (TF) technology, which enables the functionalization of several metal layer composites in a single component. The so-called hybrid machine elements can be produced by co-extrusion of aluminum and steel and subsequent die forging, heat treatment, and machining. The TF rolling bearings made by this process can provide optimized characteristics that use aluminum to reduce weight and steel for a highly loaded contact zone between a rolling element and a bearing raceway. To evaluate the applicability and the potential of this technology, theoretical investigations are presented in this paper. The stress distribution under fully flooded conditions, caused by an external load in the contact between a rolling element and the TF outer ring of an angular contact ball bearing, is analyzed statically with the finite element method. The fatigue life of the TF component can be calculated for different external axial loads and manufacturing parameters, such as steel-to-aluminum volume ratios and osculation. As a damage model, the Ioannides and Harris fatigue model and the Dang Van multiaxial fatigue criterion were used. The results show that the fatigue life has high sensitivity to the steel-to-aluminum volume ratio. For the hybrid component with a steel layer thickness of 3 mm, 90 percent of the fatigue life of pure 100Cr6 steel bearing bushings is reached. In this FE model, residual stresses due to machining processes can be regarded as an initial state, which can increase the fatigue life of this TF machine component. Full article

In the course of increasing electric mobility, the effect of electricity on parts of machines is more significant than before. Rolling bearings and their lubrication, as a part of electric motors, are subjected to harmful currents, which lead to damage in the bearing in the long term. In order to avoid such damage, the influence of the lubricant in the bearing is becoming increasingly important. The electrical behaviour of the system can be investigated by analysing the discharge currents and the breakdown voltage in rolling bearings with lubricants of different compositions. This paper presents a procedure for characterizing the breakdown voltage at the rolling bearing and the influence of conductivity of the lubricants on harmful electrical phenomena. Full article

This paper deals with a Bush plane tire rolling in critical and extreme conditions as shocks and rebounds. The approach adopted is based on previous works on the modelling of Jumbo-Jet tires. A numerical finite element model is used in the simulation of the tire. Firstly, an experimental part is dedicated to study the inner features of the tire. The tire geometry and the materials within it are described. Secondly, a 2D embedded mesh model is developed based on the tire cross-section. Then a 3D model is generated and a runway with rocks and ramps is modelled. The tire behavior while rolling over obstacles is investigated. The simulation results, such as tire deformation, are analyzed. The results show significant deformation of the tire while rolling over ramps and a low lateral stiffness, giving it a significant capacity to absorb shocks. The numerical simulation was developed in order to predict the tire behavior during landing, especially in critical and extreme conditions. Cornering simulations were realized to evaluate the self-aligning moment. The numerical simulation is an efficient tool to estimate the forces transferred to the rim axis in critical and extreme conditions. Full article

Ice friction is affected by various system and surface-related parameters such as ice temperature, ambient air temperature and humidity, relative sliding velocity, specific surface pressures and surface texture (waviness, roughness) as well as the macroscopic geometry of the samples. The influences of these parameters cannot be easily separated from each other. Therefore, ice friction is a very complex tribological system and it is challenging to draw sound conclusions from the experiments. In this work, ice friction experiments with stainless steel samples that have different isotropic surface roughness values were carried out. Two tribological experimental setups were used: (i) an inclined ice track where the sliding velocity of the freely sliding steel samples was determined and (ii) an oscillating tribometer, where the coefficient of friction was assessed. For both experimental setups, the environmental parameters such as air temperature, relative humidity and ice surface temperature as well as the test parameters such as normal load and surface pressure were kept as constant as possible. The results of the experiments are discussed in relation to the ice friction mechanisms and the friction regimes. Full article

In this work, friction loss investigations and comparisons of three different four-cylinder engines for passenger car applications are presented, using a recently developed combined approach. By merging extensive experimental with reliable and predictive journal bearing simulation results, a sub-assembly-resolved friction loss analysis of the piston group, crankshaft journal bearings and valve train is conducted for all three engines. The engines have been chosen individually based on their specific power output and crank train geometry. The measurement program covers a wide range of corresponding engine operation points (identical speed, load and thermal boundary conditions). In addition, the investigations are carried out for different engine media supply temperatures ranging from 70 ${}^{\circ }$ C to 110 ${}^{\circ }$ C for a comprehensive consideration of the friction losses at reduced lubricant viscosity. For reasons of comparability, all investigations conducted in this work have been carried out using the same modern SAE 5W30 lubricant. This is done to exclude influences from different lubricant properties which may have significant effects on the tribological behaviour of the engines’ sub-assemblies. While the diesel engine showed a friction reduction potential over the entire engine operation range when increasing the engine media supply temperatures, the gasoline engines showed a different behaviour. For the gasoline engines, disadvantages arise especially at low engine speeds. By using the developed combined approach, it was possible to assign mixed lubrication regimes at the valve train systems and at the piston groups. Full article

Physical simulations of tribo contacts in internal combustion engines can act as a supporting tool to match upcoming guidelines and emission restrictions. In particular, the scuffing resistance of the contact between the piston ring and cylinder liner suffers under decreasing oil viscosity and limitation of antiwear additives. This paper aims to provide an experimental method to simulate the scuffing of the piston ring/cylinder liner-contact and to validate this method with real engine parts and the literature from engine tests. The experimental methodology uses a linear tribometer TE77 to test specimens from original piston rings and liners under reciprocating motion. Additionally, the ring specimen is given the opportunity to perform secondary movements (ring twisting, ring turning) and to run under deficient lubrication conditions similar to the engine. A specially designed test strategy enables the reproducible creation of seizure of the tribosystem. The seizure resistance of two engine oils, tested for validation, correlates with the known engine performance. Therefore, the model test system can be seen as a reproducible tool for simulating seizure of a ring/liner-system, showing similar trends and wear mechanisms as in an engine. Surface analysis depicts similarities between the scuffed surfaces of an engine and the model and discusses the origin of seizure based on the model specimens together with the relevant literature. Full article

Rough and textured surfaces are of paramount importance for lubrication, both in nature and in technology. While surface roughness relevantly influences both friction and wear, artificial surface texturing improves the performance of slider bearings as an energy efficiency action. The simulation of hydrodynamic lubrication by taking into account complex surfaces as boundaries requires the use of computational fluid dynamics (CFD) software able to predict the pressure and the velocity profile through the thickness of the fluid and at any point within the 3D domain. In the present study, a CFD–smoothed particle hydrodynamics (SPH) code is applied to simulate hydrodynamic lubrication for a linear slider bearing in the presence of a 3D rough surface, showing the capabilities of CFD–SPH in modelling such complex interaction phenomena. Numerical assessments involve the load capacity, the 3D fields of the velocity vector, and the pressure 3D field (both within the fluid domain and at the fluid–plate interface). Full article

Due to the growing interest in robotic and haptic applications, voltage-induced friction has rapidly gained in importance in recent years. However, despite extensive experimental investigations, the underlying principles are still not sufficiently understood, which complicates reliable modeling. We present a macroscopic model for solving electroadhesive frictional contacts which exploits the close analogy to classical adhesion theories, like Johnson-Kendall-Roberts (JKR) and Maugis, valid for electrically neutral bodies. For this purpose, we recalculate the adhesion force per unit area and the relative surface energy from electrostatics. Under the assumption of Coulomb friction in the contact interface, a closed form equation for the friction force is derived. As an application, we consider the frictional contact between the fingertip and touchscreen under electrovibration in more detail. The results obtained with the new model agree well with available experimental data of the recent literature. The strengths and limitations of the model are clearly discussed. Full article