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Tribology concentrates on wear, lubrication, and friction of interacting surfaces in relative motion. Wear, which is the major reason of material dissipation, evolves in three separate stages: severe wear, steady-state, and running-in. Running-in has an important role in the loss of material performance and this process inducts the progress of the key tribological parameters. Hence, the runningin behaviour of a tribocomponent experiencing point contact in a mixed lubrication regime is inquired both experimentally and theoretically. The transient coefficient of the wear pending the running-in is predicted by using the continuum damage mechanics approach. Predictions involve the use of the load-sharing implication, taking into account the contribution of the asperities and the lubricant. The experimental work entails dynasties of pin-on-disk tests. Comparisons of the theoretical prediction and experimental tests of friction coefficient and wear coefficient are found to be in good agreement. In cases in which continuum damage mechanics--which computes the possibility that an asperity creates a wear particle and uses this data to infer a phrase for the coefficient of the wear--can forestall the volume of the wear with an error of less than 30%. [ABSTRACT FROM AUTHOR]

In this paper, the effect of adding Reduced Graphene Oxide (RGO) nano-particles to engine base oil (Poly-Alpha-Olefin-PAO) on the thermo-physical properties of nano-fluid such as viscosity, thermal conductivity, and friction coefficient, are investigated numerically and experimentally. To keep the nano-fluid stable, before using ultrasonic waves, two types of polysorbate (tween 20 and tween 80) as surfactants were added to four concentrations of 0.01, 0.02, 0.0375, and 0.05 wt%, and the effects of nano-particles were studied to find the best combination in terms of viscosity, thermal conductivity, and friction coefficient, which made the nan-fluid quite stable during the tests procedure. The results showed with good accuracy that the final nano-fluid was Newtonian and its viscosity was very similar to the base oil. An increase of 7% was observed between 25 °C and 75 °C and for the concentration of 0.02 wt%. Thermal conductivity was raised in all states with an increase in concentration and the highest effect was 8% in the concentration of 0.05 wt%. The friction tests proved a desirable decrease of as much as 45% in friction coefficient compared to base oil and 28% compared to common anti-friction material used in industry for the optimum concentration which was 0.02 wt%. In addition, experimental data were compared to the models presented in the literature and the models that could describe the behavior of this nano-fluid in the best way were reported, and an empirical equation (for each surfactant) is developed to show the variation of COF with nano-particles weight fraction in the nano-fluid. [ABSTRACT FROM AUTHOR]

Mechanical seals are mechanisms that are used to prevent fluid leakage. Since the seal surfaces are in contact with one another, hydrodynamic and contact forces are functions of surface roughness. Additionally, since the lubrication regime under specific operating conditions such as low speed or high load causes the seal to operate in the mixed lubrication regime, thus the contact of asperities plays an important role. The primary purpose of this paper is to apply the load-sharing concept to study the behavior of a mechanical seal in a mixed lubrication regime. The predicted results are compared to the published data from the literature, showing acceptable accuracy. The model presented in this paper can predict the performance of the mechanical seal system in a short execution time while providing acceptable accuracy by considering the surface roughness effect. [ABSTRACT FROM AUTHOR]

A model is presented, which enables one to predict the running-in performance of the rolling/sliding surfaces subjected to mixed-lubrication line contact. The load-sharing concept was used, in which it is assumed that both the fluid film and the asperities contribute in carrying the imposed load. The plastic deformation of asperities during the running-in is taken into consideration. In the application of the load-sharing method, it is often assumed that asperity heights have a Gaussian distribution. This assumption has been relaxed in this model. Prediction results for the variation in the arithmetic average of asperity heights ([R.sub.a]) during the running-in period for contact of two rollers are compared with published experimental data. Also presented are the results for the variation in wear volume, wear rate, and friction coefficient during the running-in period. The effect of surface pattern, speed, and load on the running-in behavior is studied. The steady-state wear rate for different surface patterns calculated from this model is compared with the wear rate predicted by the thermal desorption model, and the results are in agreement both in trend and magnitude. The effect of running-in on the Stribeck curve for different surface pattern is discussed. [DOI: 10.1115/1.4001622] Keywords: running-in, breaking-in, Stribeck curve, surface roughness pattern, mixed-lubrication, load-sharing concept, steady-state wear

In the present work, a hierarchical braided polymer composite consists of Polytetrafluoroethylene (PTFE) fibers, reinforcement fibers, and epoxy resin was designed as a self-lubricant composite and bearing. Different reinforcements such as glass, carbon, and Kevlar fibers were employed to investigate the effect of reinforcement on the wear characteristics of composites. Besides, the influence of 2D/3D braid fabric was examined on the wear behavior of samples. Also, 90 and 120 N loads were applied to assess the load impact. Results illustrated that compared to glass and Kevlar, carbon could lead to a greater wear weight loss and friction coefficient. However, PTFE tribofilm was observed according to scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDS) analysis. Moreover, with the increase of load, the wear stability of the composites faded significantly. Furthermore, the tribological features became inappropriate with the deployment of the 3D braid structure. Finally, the modeling of tribological parameters was carried out using response surface methodology-based D-optimal design. The adequacy of the models was checked by analysis of variance. Results implied that there is an excellent correlation between the model and the experiments. [ABSTRACT FROM AUTHOR]

The main goal of this study is to present a model to investigate the effect of nano-particles' weight fraction on the friction coefficient of rough contact in the mixed-lubrication regime. Experimental testing involves pin-on-disk measurements of the friction coefficient with CuO nano-particles added to engine oil. Theoretical analyses involve developing a method for treating an EHL line contact with provision for surface roughness that takes into account the load-carrying capacity of surface asperities, lubricant, and nano-particles. Results show that theoretical and experimental results for friction coefficients are in good agreement. A parametric study is conducted to investigate effect of load, the geometry of the nano-particles, and their mechanical properties as well as their weight fraction on the friction coefficient. [ABSTRACT FROM AUTHOR]

Shape memory alloys (SMA) are nowadays widely used in different industries. The two extraordinary behaviors of superelasticity and shape memory effect make these alloys a super wear-resistant material. In a range of SMA applications, contact between adjacent surfaces occurs. In this research, a formerly-developed contact model, which individually considers each asperity, is extended to cases where superelastic shape memory alloys are used. Since constitutive equations of SMAs are based on stress and strain, to establish a relationship between classical contact models and the main arguments of these constitutive equations, a representative strain based on the pseudoelastic behavior of SMAs was defined. Experiments were conducted to verify the model's predictions. In these experiments, a NiTi wire was pressed against a Steel plate; then, the measured penetration in the test and the values predicted by the contact model were compared. The reported results show an acceptable agreement between theory and experiment. [ABSTRACT FROM AUTHOR]

Elliptical bearings are one type of non-circular journal bearings which has two main advantages over the conventional circular bearings: lower temperature rise and lower vibrations. In this study, the energy equation and Reynolds equation are simultaneously solved under adiabatic boundary conditions. The predicted pressure and temperature are compared to the results of published literature for verification purposes. A parametric study is then conducted on the effect of geometrical parameters of the elliptical journal bearing on the load capacity, friction force, pressure, and oil temperature. Effect of geometric parameters of the bearing on the performance is studied. The results show that non-circularity parameter is the most influential parameter in the bearing, and an increase in the non-circularity results in the decrease in maximum pressure and temperature as well as the friction force. Increasing the eccentricity ratio, on the other hand, will cause an increase in the pressure, temperature, and the friction force. Changes in the angle between the major axis of the bearing and load direction decrease the load-carrying capacity and the non-dimensional pressure and results in an increase in the friction coefficient. [ABSTRACT FROM AUTHOR]

Equations for quantifying the subsurface shear stress in dry point contact are utilized to obtain the value and location of the maximum subsurface shear stress. A series of experiments using a pin-on-disk tribometer is conducted on run-in specimens made of steel, brass, and aluminum, and the weight loss and wear rate of the specimen are measured. The results reveal a correlation between the depth of the maximum subsurface shear stress obtained from the model and the measured wear rate. It is shown that at the onset of failure, the friction coefficient suddenly increases. This increase affects the location of maximum subsurface shear stress by pushing it toward the surface and producing wear particles. SEM images of all three friction-pair tested reveal that the size of the wear particles is directly related to the applied load. [ABSTRACT FROM AUTHOR]

Spur gears are one of the most important tools for power transmission in the industry and thus can be considered a key mechanical element. As a result of power transmission, gears might fail or experience wear and fatigue. So the improvement in the design and modification of tooth profile of gears can significantly reduce friction loss and wear of the gear teeth and therefore it increases the useful life, improves the quality, and reduces the cost. The purpose of this study is to show the influence of addendum modification on the tribological performance of spur gears which are operating in the mixed elastohydrodynamic lubrication. In this paper, a model based on the load-sharing concept is employed to study the effect of changing addendum modification on the performance of spur gears, the amount of wear rate, and the lubricant film thickness. To this end, a model based on the load-sharing concept is employed which takes the geometry and lubricant properties as input and predicts the friction coefficient, load carried by fluid film and asperities, efficiency, and wear rate as output. [ABSTRACT FROM AUTHOR]

Aspherical lenses are produced using glass molding technology in a temperature close to its glass transition temperature. Friction and wear between the moving mold surface and the heated glass is one of the important aspects of the optical lenses production. This paper presents an experimental study on the tribological behavior of BK7 optical glass at elevated temperatures using pin-on-disk test rig. Thirty experiments have been conducted to study the effect of operating temperature, applied load, and sliding speed on the friction coefficient and wear rate of BK7 optical glass disks in contact with alumina pin. Also, the relation between the generated entropy and the weight loss under different operating conditions has been studied. The experimental results show that with increasing the temperature up to the glass transition temperature, the wear volume and friction coefficient increase. Further increase in the temperature results in a decrease in the wear volume and friction coefficient. Increasing the applied load results in an increase in the wear rate. An investigation on the correlation of wear rate and the dissipated power shows a linear relation between these two parameters. [ABSTRACT FROM AUTHOR]

Wear coefficient and friction coefficient are two of the key parameters in the performance of any tribo-system. The main purpose of the present research is to use continuum damage mechanics to predict wear coefficient. Thus, a contact model is utilized that can be used to obtain the friction coefficient between the contacting surfaces. By applying this model to the continuum damage mechanics model, the wear coefficient between dry surfaces is predicted. One of the advantages of using this model is that the wear coefficient can be numerically predicted unlike other methods which highly rely on experimental data. In order to verify the results predicted by this model, tests were performed using pin-on-disk test rig for several ST37 samples. The results indicated that the wear coefficient increases with increasing the friction coefficient. [ABSTRACT FROM AUTHOR]