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Applications

J. Tribol. 2012;134(4):041101-041101-15. doi:10.1115/1.4006980.

The piston/cylinder interface of swash plate–type axial piston machines represents one of the most critical design elements for this type of pump and motor. Oscillating pressures and inertia forces acting on the piston lead to its micro-motion, which generates an oscillating fluid film with a dynamically changing pressure distribution. Operating under oscillating high load conditions, the fluid film between the piston and cylinder has simultaneously to bear the external load and to seal the high pressure regions of the machine. The fluid film interface physical behavior is characterized by an elasto-hydrodynamic lubrication regime. Additionally, the piston reciprocating motion causes fluid film viscous shear, which contributes to a significant heat generation. Therefore, to fully comprehend the piston/cylinder interface fluid film behavior, the influences of heat transfer to the solid boundaries and the consequent solid boundaries’ thermal elastic deformation cannot be neglected. In fact, the mechanical bodies’ complex temperature distribution represents the boundary for nonisothermal fluid film flow calculations. Furthermore, the solids-induced thermal elastic deformation directly affects the fluid film thickness. To analyze the piston/cylinder interface behavior, considering the fluid-structure interaction and thermal problems, the authors developed a fully coupled simulation model. The algorithm couples different numerical domains and techniques to consider all the described physical phenomena. In this paper, the authors present in detail the computational approach implemented to study the heat transfer and thermal elastic deformation phenomena. Simulation results for the piston/cylinder interface of an existing hydrostatic unit are discussed, considering different operating conditions and focusing on the influence of the thermal aspect. Model validation is provided, comparing fluid film boundary temperature distribution predictions with measurements taken on a special test bench.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):041102-041102-11. doi:10.1115/1.4007246.

This paper describes the robust optimum design considering dimensional tolerances for fluid dynamic bearings (FDBs) of 2.5 in. hard disk drives (HDDs). Recently, 2.5 in. HDDs are widely used for mobile devices such as laptops, video cameras, and car navigation systems. Therefore, in mobile devices, high durability toward external vibrations is essential for high HDD performance. On the other hand, FDBs for HDD spindle motors are generally manufactured by mass production processes which will eventually require reduction of production costs. Consequently, the FDBs are demanded to be easily manufactured and expected to have an insensitive design with low variability of bearing characteristics due to manufacturing errors. In this paper, first, the vibration model of the spindle motor is constructed, and then the vibration experiment was carried out in order to verify the appropriateness of the vibration model. Second, the bearing characteristics are calculated considering dimensional tolerance using optimum design combined with the statistical method in which the dimensional tolerance is assumed to distribute according to the Gaussian distribution. The bearing characteristics are estimated by expectation and standard deviation. Finally, the results of this robust optimum design compared with ones of optimum design neglecting tolerance, and the validity of this technique, were clarified. It was found from the results that the tolerances of radial clearance and groove depth are important factors to be considered to reduce the variability of the amplitude and friction torque. In addition, the variability of the amplitude strongly depends on the groove depth.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):041103-041103-8. doi:10.1115/1.4003764.

Taking the water pump bearing with one roller row (WR)-type auto water pump bearing as a research sample, an analytical calculation method is developed to improve the accuracy and efficiency of the current calculations for the bearing loads and life in engineering application. Considering the misalignment due to the deflection of the bearing spindle, the bearing internal loads and deformations under the action of the complicated external space loads are obtained. The bearing fatigue life including the lives of the rollers and the balls is also calculated with considering the non-normal load distribution caused by the spindle deflection and the roller tilt. The bearing load and life calculation results are compared with those calculated by the traditional method in which the deflection of the bearing spindle and the roller tilt are ignored. The effects of the bearing spindle deflection on the load distribution and the life of the auto water pump bearing are analyzed and discussed. The life decrease in the auto water pump bearing is significant due to the deflection of the bearing spindle and it is recommended to give more attention to this deflection for the high quality of the bearing design and calculation.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):041104-041104-7. doi:10.1115/1.4007346.

Understanding the tribological interactions between shoe and floor materials is important in order to enhance shoe and floor design and to prevent slip and fall accidents during walking. In the present investigation, experiments were conducted using a custom developed pin-on-disk type tribometer to understand the influence of boundary and hydrodynamic properties on the shoe-floor materials’ coefficient of friction. Specifically, polyurethane shoe material was slid against vinyl floor material in the presence of varying lubricants (i.e., water, detergent, three diluted glycerol concentrations, and canola oil). The experiments were conducted for a range of biologically relevant sliding velocities from 0.05 m sec−1 to 1.0 m sec−1 at a contact pressure of 266.1 kPa under ambient conditions. The fluid chemical composition appeared to affect the boundary friction coefficient with longer-chain molecules resulting in a decreased coefficient of friction. As fluid viscosity increased, the rate of coefficient of friction decay increased with respect to increasing fluid entrainment velocity, suggesting less material contact and increased film thickness. The nondimensional film thickness under all conditions was calculated and the nondimensional film thickness consistently increased with increased viscosity and speed. Additionally, the effect of functionally achievable variations in polyurethane shoe roughness on the coefficient of friction was examined and found to have no statistically significant effect on boundary or hydrodynamic contributions to the coefficient of friction.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):041105-041105-9. doi:10.1115/1.4007349.

The stiffness of rolling bearings and their supporting structure in wind turbines is very low because of their large dimensions. Accurate predictions of the deformations of bearing elements and their flexible supports are difficult. In this paper, the finite element method (FEM) is employed to determine the mechanical behaviors of the large-scale bearings. Traction springs are used to model the contact between raceways and rolling elements. A pitch bearing supports the whole hub, the partial hub and the rigid outer ring under loads is studied. Results show that a comprehensive model representing the surrounding structure is necessary for an accurate assessment of the actual loading conditions of large-scale WTG pitch bearings. The load is shared well between the two bearing rows by appropriately decreasing the stiffness of the plane rib.

Commentary by Dr. Valentin Fuster

Contact Mechanics

J. Tribol. 2012;134(4):041401-041401-10. doi:10.1115/1.4007219.

The accuracy of the fatigue life calculations in rolling bearing simulations is highly dependent on the precision of the roller-raceway contact simulations. Several different methods exist to simulate these pressure distributions and in time domain bearing simulations, where many contacts need evaluation, the simple and time efficient methods are more popular, yielding erroneous life estimates. This paper presents a new six degree of freedom frictionless quasi-static time domain cylindrical roller bearing model that uses high precision elastic half-space theory to simulate the contact pressures. The potentially higher computational demand using the advanced contact calculations is addressed by preprocessing a series of contacts at different centerline approaches and roller tilt angles, which are used for interpolating contact results during time domain simulations. It is demonstrated that this new model allows for simulation of bearing misalignments, roller centrifugal forces, and flange contact induced roller tilt moments, and that the effect of these conditions is directly evaluated in a detailed fatigue life analysis. Finally, the stiffness of the bearing model is validated against existing experimental data with good correlation.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):041402-041402-8. doi:10.1115/1.4007270.

This paper presents a finite element approach for modeling a thermal-electrical-mechanical coupled-field contact comprised of an elastic hemisphere pressed against an elastic half-space. The goal of this investigation is to develop a fundamental understanding of the behavior of this multiphysics contact, with a particular interest on the contact area through which current flows. The results from the model illustrate a distinct difference in contact behavior between force control and displacement control in the presence of an applied electrical potential/current. It is shown that, while Hertz contact theory can be used to accurately predict the behavior of the contact under force control, a new relationship is established to accurately predict the behavior of the contact under displacement control.

Commentary by Dr. Valentin Fuster

Elastohydrodynamic Lubrication

J. Tribol. 2012;134(4):041501-041501-7. doi:10.1115/1.4007107.

Lubricant flow properties of polyalphaolefin (PAO) oil have been experimentally investigated based on a ball-on-disc configuration under micro oil supply condition. The oil pool shape and central film thickness in the contact region were obtained using fluorescence microscopy and optical interferometry, respectively. It has been found that the relative length between the inlet meniscus and Hertzian center point in the oil pool to Hertzian radius was much larger than 1 in a smaller lubricant supply of 20 μl, and the corresponding contact region initially entered the elastohydrodynamic lubrication (EHL) region and then became starved with the increasing speed. The variations of the relative film thickness as a function of starvation degree and the ratio of relative length to Hertzian radius were proposed to explain the obtained results. Besides, the fluorescence technique was used to directly observe the inlet meniscus position of the oil pool and helped to gain more understanding of the lubricant flow properties under micro oil supply condition.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):041502-041502-12. doi:10.1115/1.4007348.

In this investigation, a new approach was developed to study the influence of cage flexibility on the dynamics of inner and outer races and balls in a bearing. A 3D explicit finite element model (EFEM) of the cage was developed and combined with an existing discrete element dynamic bearing model (DBM) with six degrees of freedom. The EFEM was used to determine the cage dynamics, deformation, and resulting stresses in a ball bearing under various operating conditions. A novel algorithm was developed to determine the contact forces between the rigid balls and the flexible (deformable) cage. In this new flexible cage dynamic bearing model, the discrete and finite element models interact at each time step to determine the position, velocity, acceleration, and forces of all bearing components. The combined model was applied to investigate the influence of cage flexibility on ball-cage interactions and the resulting ball motion, cage whirl, and the effects of shaft misalignment. The model demonstrates that cage flexibility (deflection) has a significant influence on the ball-cage interaction. The results from this investigation demonstrate that the magnitude of ball-cage impacts and the ball sliding reduced in the presence of a flexible cage; however, as expected, the cage overall motion and angular velocity were largely unaffected by the cage flexibility. During high-speed operation, centrifugal forces contribute substantially to the total cage deformation and resulting stresses. When shaft misalignment is considered, stress cycles are experienced in the bridge and rail sections of the cage where fatigue failures have been observed in practice and in experimental studies.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):041503-041503-10. doi:10.1115/1.4006277.

In the classical Reynolds equation-based modeling of lubrication, the exit area is only considered through a pressure boundary condition which fails to predict the remaining amount of lubricant on each moving surface after the film rupture. A two-phase flow model using the Navier-Stokes equations and a diffuse interface approach is developed to analyze the lubricant behavior at the exit of rolling and sliding lubricated line contacts. After physical and numerical descriptions of the two-phase flow model, results are compared with experimental data from the literature. Good agreements are found concerning pressure profiles and meniscus exit abscissas. The model is then used to study in detail the flow behavior at the exit for different surface tensions. It is shown that when surface tension effects are important, recirculation areas occur downstream the air/oil meniscus. Sliding effects on fluid distribution are then investigated. Finally, an analytical approach is proposed, as a synthesis of the numerical results.

Commentary by Dr. Valentin Fuster

Friction & Wear

J. Tribol. 2012;134(4):041601-041601-7. doi:10.1115/1.4006577.

Friction-induced instabilities can be caused by different separate mechanisms such as elastodynamic or thermoelastic. This paper suggests another type of instability due to the temperature dependency of the coefficient of friction. The perturbations imposed on the surface temperature field during the frictional sliding can grow or decay. A stability criterion is formulated and a case study of a brake disk is performed with a simple model without including effects of transforming layer and chemical/physical properties change with temperature. The disk is rigid and the coefficient of friction depends on temperature. We show that the mechanism of instability can contribute to poor reproducibility of aircraft disk brake tests reported in the literature. We propose a method to increase the reproducibility by dividing the disk into several sectors with decreased thermal conductivity between the sectors.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):041602-041602-7. doi:10.1115/1.4007016.

New applications of carbon-based materials have been continuously developed in recent years. Carbon nanofibers (CNFs) with silane coatings were added into high density polyethylene (HDPE) to improve the tribological properties of the nanocomposite material. The nanocomposites were fabricated with various weight percentages of carbon nanofibers (0.5 wt.%, 1 wt.% and 3 wt.%) that were treated with different silane coating thicknesses (2.8 nm and 46 nm) through melt-mixing and compressive processing. The wear and friction tests were performed on a pin-on-disc tribometer under phosphate buffered saline lubricated condition. Compared with the neat HDPE, the friction coefficients of the nanocomposites were reduced in all samples, yet only a couple of nanocomposite samples showed lower wear rates. Micro-hardness measurements of the nanocomposites were carried out and CNFs were found to be capable of increasing the material’s micro-hardness. The effects of concentration and silane coating thickness of CNFs on the tribological properties of the resulting nanocomposites were analyzed and the wear mechanisms of the HDPE/CNF nanocomposites were discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):041603-041603-11. doi:10.1115/1.4007218.

The wear-fatigue rupture of Ni88 P 11.78 Co0.12 Fe0.10 (NiP) and Ni80.55 Cr15.25 B4.20 (NiCrB) glasses prepared by planar–flow casting have been studied using a test under simultaneous constant and cyclic loading generated by an eccentric rotation ceramic antagonist. For better apprehending the phenomena related to the structural state changes of samples before and after tests, structural characterization by x-ray diffraction, mechanical characterization by measuring Vickers microhardness (HV 0.1) and chemical composition by X-ray photoelectron spectroscopy (XPS) analysis have been carried out on as-quenched and worn dull side ribbons. Rupture surfaces, in S–N curves, have been measured by scanning electron microscope. Wear-fatigue contact tests consist to impose, simultaneously, a traction strain and cyclic normal stresses which generate traction, compression, rolling, bending and shearing. All results obtained from the two selected glasses (NiP and NiCrB) are systematically compared with those of a nickel pure crystalline foil (Ni). We evaluate mainly the wear mechanism, the mode and the typical rupture surface observed in NiP, NiCrB and Ni specimens. We specify the conditions of obtaining these rupture surfaces which often present in smooth plane, veining and “chevrons” patterns. All results show a great wear and fatigue resistance for the two metallic glasses compared to Ni. The NiCrB wear resistance is superior to that of NiP, while the difference in their fatigue limit is not clearly distinct. The reasons for the differences in wear and fatigue behavior will be discussed in relation to the metallic glass thermal stability, chemical composition, microhardness and surface rupture topography.

Commentary by Dr. Valentin Fuster

Hydrodynamic Lubrication

J. Tribol. 2012;134(4):041701-041701-7. doi:10.1115/1.4007108.

In recent years the efforts to better control friction and wear have focused on surface-topography modification through surface texturing. Although a lot of effort, including experimental and analytical work, has been put into finding the optimal texturing parameters and design rules for reduced friction, optimization is still too often limited and based on a trial-and-error approach. Therefore, the aim of the present research work was to investigate the possibility of using kurtosis and skewness as the design parameters for selecting the optimal texturing pattern for contact surfaces operating under lubricated conditions. The results of this investigation performed on groove- and dimple-textured surfaces under low-load, low-sliding speed conditions confirmed the correlation between the kurtosis and skewness parameters and the coefficient of friction. For textured surfaces an increase in the kurtosis and a more negative skewness, obtained by reducing the cavity size, increasing the cavity depth and decreasing the texturing density, were found to yield a lower friction. Furthermore, kurtosis and skewness were recognized as suitable parameters for the optimization of textured surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):041702-041702-8. doi:10.1115/1.4007347.

In this work, the performance of barrel-shaped laser-textured piston rings is numerically investigated. The surface texture, parameterized by the dimple density, dimple depth, and dimple distribution pattern, is optimized to minimize the friction coefficient for piston rings of variable curvature. We consider fully textured as well as partially textured piston rings with two different dimple distributions patterns: a central dimple distribution, and a distribution along the piston ring edges. Finally, the sensitivity of the optimal surface parameters to the piston ring curvature is assessed.

Commentary by Dr. Valentin Fuster

Tribochemistry & Tribofilms

J. Tribol. 2012;134(4):042301-042301-6. doi:10.1115/1.4006994.

Even when the charging of lapping plates can extensively influence their subsequent finishing performance, the subject has been scarcely treated in specialized literature. The present paper aims to help fill such a gap and gain a better insight of the charging process. A semiquantification of the diamond particles integrated into the lapping plate surface as a function of charging time was performed by Raman spectroscopy and scanning electron microscopy, together with a simple image-analysis procedure. The corresponding evolution of surface rough features was followed from atomic force micrographs with the aid of fractal-analysis tools. It was observed that charging proceeds in two stages, both with different rates of diamond particle integration. This leads to a significant waste of diamond slurry. During the first stage, the charging ring seems to preferentially promote a further flattening of the lapping-plate surface. Diamond particles are apparently more readily incorporated into such “flattened” regions during the second stage. The results suggest a specific topographic condition must be attained before the diamond can be efficiently integrated into the lapping plate surface. A lapping-plate preconditioning step could help improve this situation and reduce the amount of abrasive waste during charging.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Tribol. 2012;134(4):044501-044501-5. doi:10.1115/1.4007109.

This paper presents an experimental procedure to evaluate the load-carrying capacity of a fixed-incline slider bearing (dimensionless load W versus convergence ratio K) using a slider-on-disk lubricating film test rig. In general, the applied load is the dependent variable and is directly measured for different convergence ratios such that the relation of the load-carrying capacity W and the convergence ratio K can be obtained. The load and slider inclination are fixed in the present approach, and the film thickness is measured at different speeds. As the dimensionless load can be a function of speed and film thickness, the variation of load-carrying capacity with respect to speed can be obtained even under a constant load and a fixed incline. It is shown that the measured load-carrying capacity is lower than that predicted by the classical hydrodynamic theory. Nevertheless, the experimental results acquire the same trend in the variation of dimensionless loads with convergence ratios. The theory holds that the load-carrying capacity is a single function of the convergence ratio. However, the experimental results show that the dimensionless load-carrying capacity is affected by the inclination angle of the slider, load, and the properties of lubricating oils.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;134(4):044502-044502-5. doi:10.1115/1.4007350.

The present theoretical analysis investigates the simultaneous effect of lubricant inertia and non-Newtonian pseudoplastic lubricant (lubricant blended with viscosity index improver and viscosity thickener)–Rabinowitsch fluid model on the performance of externally pressurized annular hydrostatic thrust bearings. A close form solution is obtained for pressure distribution. The effect of centrifugal inertia on the pressure distribution in the recess region is considered by taking non-constant recess pressure under a hydrodynamic condition. The load capacity and flow rate have been numerically calculated for various values of viscosity index improver together with the centrifugal inertia effects. In the limiting case in which there is an absence of pseudoplasticity, the results are compared with the pre-established Newtonian lubricants and are found to be in good agreement.

Commentary by Dr. Valentin Fuster

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