Search Results (9)

For wind turbine applications, there is a cyclic load-varying process between rolling elements and raceways in pitch bearings. This kind of motion can also lead to radial fretting. However, this is seldom addressed under grease-lubricated conditions in the literature. In this study, grease-lubricated point contact problems have been investigated experimentally under cyclic load-varying conditions. The findings revealed that as the load-varying range diminishes, the variation in grease film distribution becomes more subtle and the rate of discharge of thickener fiber clusters in the stick zone decelerates. This is due to the fact that the rate of change in the Hertz contact radius is reduced and the migration of grease is weakened during the unloading process. Due to the large apparent viscosity of grease with a high soap content, entrapped grease is not easily discharged during loading, and the thickness of the film in the stick zone progressively increases as the soap content of the grease is augmented. This also causes the variable load zone to wear out more easily. As the grease is subjected to repeated loading and unloading, there is a gradual reduction in film thickness, and larger thickener fiber clusters tear, resulting in a flattened form and shear thinning. Grease containing sulphur–phosphorus additives demonstrates a superior effect on reducing fretting wear within the large variable load range but generally proves effective for smaller load-varying ranges. This study may offer insights into the degradation of grease under variable load motion and methods to prevent radial fretting wear. Full article

Dental decay still presents a major health problem among children. Its treatment usually requires the use of stainless steel crowns. This study compares the wear behavior of 316 L stainless steel and polyetheretherketone (PEEK) composite under identical test conditions. The wear tests were conducted in a reciprocating ball-on-plate tribometer (Plint TE67/R) using alumina balls as a counterface and artificial saliva as a lubricant at 37 °C to faithfully mimic oral conditions. The coefficient of friction (COF) and specific wear rate (k) values were determined and SEM/EDS examinations were performed to identify the predominant wear mechanisms. Results showed that PEEK exhibited a significantly lower coefficient of friction (COF = 0.094 ± 0.004) and thus lower wear volume (ΔV = 0.0078 ± 0.0125 mm³) and higher wear resistance, with an average value of specific wear rate of k = 9.07 × 10⁻⁶ mm³N⁻¹m⁻¹ when compared to stainless steel (COF = 0.32 ± 0.03, ΔV = 0.0125 ± 0.0029 mm³, k = 1.45 × 10⁻⁵ mm³N⁻¹m⁻¹). PEEK was revealed to be a potential material for use in pediatric crowns due to its high wear resistance while overcoming the disadvantages associated with steel at both an aesthetic and biological level. Full article

The squeeze film effect was discussed in several fields, but mostly under the same pressure boundary conditions. However, pressures at the inlet and outlet are different for aerostatic bearings. In this paper, the dynamic Reynolds equation group, with the stiffness and damping pressure written separately, is deducted and numerically solved with a high-pressure boundary for a parallel flat and circular thin film. The circular thin film considers the two results of the supply pressure boundary inside and outside. All dynamic pressure distribution and stiffness curves are given in a dimensionless form, and a comparative analysis of squeeze film characteristics with and without external pressure is conducted. From the calculation results, it can be concluded that the squeeze effect shows damping for zero-frequency and stiffness for infinite-frequency for compressible lubricants. The dynamic pressure in the static high pressure region is also high at high frequencies affected by gas compressibility. Based on these analytical results, the transfer functions of the thin film are given to further analyze the dynamic performance of aerostatic bearings, and the shape of the response curve approximates an exponential decay form, even when the amplitude increases to 10% of the gas film thickness. Full article

Reaction wheels (RW), the most common attitude control systems in satellites, are highly prone to failure. A satellite needs to be oriented in a particular direction to maneuver and accomplish its mission goals; losing the reaction wheel can lead to a complete or partial mission failure. Therefore, estimating the remaining useful life (RUL) over long and short spans can be extremely valuable. The short-period prediction allows the satellite’s operator to manage and prioritize mission tasks based on the RUL and increases the chances of a total mission failure becoming a partial one. Studies show that lack of proper bearing lubrication and uneven frictional torque distribution, which lead to variation in motor torque, are the leading causes of failure in RWs. Hence, this study aims to develop a three-step prognostic method for long-term RUL estimation of RWs based on the remaining lubricant for the bearing unit and a potential fault in the supplementary lubrication system. In the first step of this method, the temperature of the lubricants is estimated as the non-measurable state of the system using a proposed adjusted particle filter (APF) with angular velocity and motor current of RW as the available measurements. In the second step, the estimated lubricant’s temperature and amount of injected lubrication in the bearing, along with the lubrication degradation model, are fed to a two-step particle filter (PF) for online model parameter estimation. In the last step, the performance of the proposed prognostics method is evaluated by predicting the RW’s RUL under two fault scenarios, including excessive loss of lubrication and insufficient injection of lubrication. The results show promising performance for the proposed scheme, with accuracy in estimation of the degradation model’s parameters around 2–3% of root mean squared percentage error (RMSPE) and prediction of RUL around 0.1–4% error. Full article

The article describes the results of experimental studies of electrorheological (ER) properties of lubricating oils containing an admixture of an ionic liquid as the electrically active ingredient. The novelty of these studies consists of the use of selected ionic liquids as additives to hydrocarbon oils in order to obtain quasi-homogenous mixtures with electrorheological properties. So far, such studies have not been carried out. Basic research, which consisted in determining the rheological characteristics in the presence of an external direct electric field, was carried out on a specially designed and built stand, which used a modified Brookfield DV-III Ultra viscometer. The conducted research showed that the produced mixtures generated the ER effect in the presence of a direct electric field with an intensity of up to 0.2 kV·mm⁻¹. The tested mixtures showed different electrorheological characteristics. The research was also carried out in the so-called dielectric spectroscopy using the Hewlett Packard HP4192A impedance analyzer. The mechanism of generating and decaying the ER effect was diagnosed by in situ microscopy using the Nikon Eclipse LV100D optical microscope. It was found that the course of the τ = f($\dot{\gamma }$) characteristic of a mixture of hydrocarbon oil with a small admixture of an ionic liquid is mainly influenced by the so-called dielectric properties of the electrically active component, or rather their change as a function of the applied BIAS (DC) voltage. At the same time, the obtained results of the research gave grounds to state that the electrorheological characteristics also depend on many physicochemical properties of the mixture components and on the differentiation of their values e.g., from the difference in viscosity of the insulating base oil and the added ionic liquid, and also from the difference in the value of the surface tension of the base oil and the added ionic liquid. In these studies, it was found that the surface tension of the CJ001 ionic liquid at 25 °C was 26.032 mN·m⁻¹. The surface tension of CJ008 was 28.099 mN·m⁻¹ and that of PAO-6 oil was almost the same, i.e., 27.523 mN·m⁻¹. The first mixture (GP1 + CJ001) showed Bigham characteristics and the second (PAO6 + CJ008) Newtonian, in the second mixture, the viscosity difference of the components was two times lower than in the first one (GP1—12.61 mPa·s, CJ001—552.42 mPa·s and PAO6—47.35 mPa·s, CJ008—327.24 mPa·s). Full article

Here, for non-breaking short surface waves, we have experimentally determined the value of the turbulent eddy viscosity ${\nu }_T$ or its ratio ${\nu }_T^{*}\equiv {\nu }_T/\nu$, where $\nu$ is the water kinematic viscosity. The non-breaking wave-generated turbulent eddy viscosity ${\nu }_T$ was found to depend on the ratio of the wave period, T, to the microscale Kolmogorov time scale, ${\tau }_{\eta }$. Our observations were consistent with ${\nu }_T^{*}=1.46·{(T/{\tau }_{\eta })}^{-2.6}$ when $(T/{\tau }_{\eta })<0.9$. That implied that the ${\nu }_T^{*}\propto {ϵ}^{-1.3}$, where $ϵ$ is the background turbulent energy dissipation rate. The near-surface turbulent flow associated with non-breaking waves was characterized by a short inertial subrange. The background turbulence appears to modulate the amount of energy the non-breaking waves dissipate locally and, consequently, the wave’s decay rate. Our results imply that the background turbulent flow acts as a lubricant, permitting waves to propagate further when traveling over a more energetic turbulent background flow. Our results have implications for the modeling of oceanic wave propagation or the air–sea exchange processes. Full article

Graphene, as a rising-star materials, has attracted interest in fabricating lightweight self-lubricating metal matrix composites with superior mechanical and wear properties. In this work, graphene nanoplatelets (GNPs) reinforced AZ31 alloy composites were fabricated by a powder metallurgy technique and then a hot extrusion. The effects of GNPs content (0.5, 1.0, and 2.0 wt.%) on the microstructures, mechanical properties, and wear performance of the extruded GNPs/AZ31 composites were studied. It was found that the addition of GNPs resulted in a weakened basal plane texture and grain refinement of the AZ31 matrix metal. Less than 1.0 wt.% GNPs in GNPs/AZ31 composites resulted in the enhancement in both Vickers hardness and tensile yield strength with acceptable elongation. The Vickers hardness and tensile yield strength of 1.0GNPs/AZ31 composite increased by 4.9% and 9.5% respectively, compared with the unreinforced AZ31. Moreover, the elongation of the composites was about the same as the AZ31 base alloy. Both the friction coefficient and the wear mass loss continuously decreased with the increasing GNPs content, which exhibited a self-lubricating effect. The relationship of the friction coefficient and wear mass loss with the GNPs content could be modeled in terms of the Holliday model and the exponential decay model, respectively. The worn surface morphology revealed that adhesive wear and abrasive wear simultaneously acted in AZ31 alloy. Nevertheless, abrasive wear became the dominant wear mechanism in the GNPs/AZ31 composites. Full article

Polymer brushes are outstanding lubricants that can strongly reduce wear and friction between surfaces in sliding motion. In recent decades, many researchers have put great effort in obtaining a clear understanding of the origin of the lubricating performance of these brushes. In particular, molecular dynamics simulations have been a key technique in this scientific journey. They have given us a microscopic interpretation of the tribo-mechanical response of brushes and have led to the prediction of their shear-thinning behavior, which has been shown to agree with experimental observations. However, most studies so far have focused on parallel plate geometries, while the brush-covered surfaces might be highly curved in many applications. Here, we present molecular dynamics simulations that are set up to study the friction for brushes grafted on the exterior of cylinders that are moving inside larger cylinders that bear brushes on their interior. Our simulations show that the density distributions for brushes on the interior or exterior of these cylinders are qualitatively different from the density profiles of brushes on flat surfaces. In agreement with theoretical predictions, we find that brushes on the exterior of cylinders display a more gradual decay, while brushes on the interior of cylinders becomes denser compared to flat substrates. When motion is imposed, the density profiles for cylinder-grafted brushes adapt qualitatively differently to the shear motion than observed for the parallel plate geometry: the zone where brushes overlap moves away from its equilibrium position. Surprisingly, and despite all these differences, we observe that the effective viscosity is independent of the radius of the brush-grafted cylinders. The reason for this is that the viscosity is determined by the overlap between the brushes, which turns out to be insensitive to the exact density profiles. Our results provide a microscopic interpretation of the friction mechanism for polymer brushes in cylindrical geometries and will aid the design of effective lubricants for these systems. Full article

The service life of rolling bearings is significantly affected by the lubricating film formation in elastohydrodynamic (EHD) contacts. Grease lubricated EHD contacts show a film thickness decay from a characteristic rotational speed, which is referred to as starvation. Thus, the film thickness of grease lubricated contacts differs from that of oil lubricated contacts. However, the base oil properties under fully flooded conditions are commonly assumed to estimate the operating lifetime of grease lubricated bearings, which are usually not fully flooded. Hence, this assumption results in an overestimation of the film thickness for rotational speeds in the range of starvation, which can lead to uncertainties in the bearing design. At high rotational speeds, i.e., high over rolling frequencies, starvation is likely to occur, due to insufficient lubricant supply by replenishment behind the rolling element. Therefore, the focus of this contribution is to investigate the effect of over rolling frequency, and thus replenishment time, on the lubricating film formation in starved, grease lubricated EHD contacts. The film thickness measurements were performed on a ball-on-disc tribometer, which was extended by adding a second ball specimen in front of the measuring ball. By varying the angular distance between the two contacts, the lubricant displacement can be controlled, such that the effect of replenishment time on the film formation can be determined. These investigations should help to establish an advanced understanding of the mechanisms of grease lubrication, and encourage future work with a focus on developing a method to predict the film formation in grease lubricated EHD contacts. Full article