Research Papers

J. Tribol. 2012;135(1):011001-011001-9. doi:10.1115/1.4007537.

Tungsten sulfide is a transition metal dichalcogenide (TMD) with excellent self-lubricating properties, and a potential candidate for coatings for MEMS applications. Its mechanical and tribological properties can be further improved by alloying it with carbon (W-S-C films). These films are commonly manufactured by sputter deposition. The present work investigates the influence of sputtering procedure on the microtribological performance of W-S-C films. For this purpose, carbon was incorporated in the films via three different ways: (1) by using a reactive gas (CH4); (2) by co-sputtering from two separate targets (WS2 and C); and (3) by sputtering from a composite target of graphite embedded with WS2 pellets. The films were characterized with scanning electron microscopy (SEM), nanoindentation, atomic force microscopy (AFM), and micro-Raman spectroscopy (RS). Reciprocating wear tests were performed on a microtribometer with steel balls as counterbodies. The worn surfaces were investigated with white light confocal microscopy, RS, and X-ray photoelectron spectroscopy (XPS). The results show that the total wear decreases with the hardness of the investigated films and increases with applied load of the tribological test. The friction coefficient at higher load is governed by the roughness of the films. At low load, the presence of graphitic carbon determines the friction coefficient. No transfer of material from the counteracting body is observed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;135(1):011101-011101-15. doi:10.1115/1.4007606.

Cold rotary forging is an advanced but complicated metal forming technology with continuous local plastic deformation. Investigating the wear is significant for effectively predicting the life of the dies and improving the workpiece surface quality. This paper is aimed to use the FE method to predict the wear response over the surfaces of the dies and the workpiece in cold rotary forging. For this purpose, a 3D elastic-plastic dynamic explicit FE model of cold rotary forging of 20CrMnTi alloy is developed using the FE software ABAQUS/Explicit and its validity is verified theoretically and analytically. Based on this valid 3D FE model, a systematic study has first been conducted, modeling and explaining the contact pressure and slip distance response. Then, the wear response that occurs at the surfaces of the dies and the workpiece is achieved. Finally, the effect of the process parameters, rotational speed n of the upper die, feed rate v of the lower die, outer/inner diameter of the ring workpiece, on the wear response is revealed. The results of this research help us better understand the complicated wear mechanisms in cold rotary forging. Moreover, the modeling methods proposed in this paper have the general significance to study the wear problems in other complicated metal forming processes.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;135(1):011501-011501-9. doi:10.1115/1.4007693.

The paper describes results obtained from the micro-elastohydrodynamic lubrication (micro-EHL) modeling of the gear tooth contacts used in micropitting tests together with a contact fatigue and damage accumulation analysis of the surfaces involved. Tooth surface profiles were acquired from pairs of helical test gears and micro-EHL simulations were performed corresponding to surfaces that actually came into contact during the meshing cycle. Plane strain fatigue and damage accumulation analysis shows that the predicted damage is concentrated close to the tooth surfaces and this supports the view that micropitting arises from fatigue at the asperity contact level. A comparison of the micropitting performance of gears finish-ground by two alternative processes (generation-grinding and form-grinding) suggests that 3D “waviness” may be an important factor in explaining their different micropitting behavior.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;135(1):011601-011601-7. doi:10.1115/1.4007675.

In an effort to study the role of strain rate response on the tribological behavior of metals, room temperature experiments were conducted by sliding commercially pure titanium and a-iron pins against an H-11 die steel flats of various surface textures. The steel flat surface textures were specifically prepared to allow for imposing varying amounts of strain rates at the contacting interface during sliding motion. In the experiments, it was observed that titanium (a harder material than iron) formed a transfer layer on H-11 steel surface textures that produced higher strain rates. In contrast, the titanium pins abraded the steel surfaces that produced lower strain rates. The iron pins were found to abrade the H-11 steel surface regardless of the surface texture characteristics. This unique tribological behavior of titanium is likely due to the fact that titanium undergoes adiabatic shear banding at high strain rates, which creates pathways for lower resistance shear planes. These shear planes lead to fracture and transfer layer formation on the surface of the steel flat, which ultimately promotes a higher strain rate of deformation at the asperity level. Iron does not undergo adiabatic shear banding and thus more naturally abrades the surfaces. Overall, the results clear indicated that a materials strain rate response can be an important factor in controlling the tribological behavior of a plastically deforming material at the asperity level.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;135(1):012001-012001-5. doi:10.1115/1.4007676.

The oxidized multiwalled carbon nanotubes (MWCNTs) were modified with stearic acid (SA) molecules. The SA-modified MWCNTs were characterized with scanning electron microscopy, transmission electron microscopy, and Fourier transform-infrared spectroscopy. The tribological properties of the oxidized and SA-modified MWCNTs as additives in water were comparatively investigated with a four-ball tester. The results showed the SA-modified MWCNTs in water have better tribological properties including friction reduction and antiwear than oxidized MWCNTs. The possible mechanism of SA-modified MWCNT as an additive in water was discussed. This research provides the opportunity for the lubricant application of MWCNTs.

Commentary by Dr. Valentin Fuster


J. Tribol. 2012;135(1):011102-011102-11. doi:10.1115/1.4007806.

Rolling element bearing damage detection is one of the foremost concerns in rotating machinery. The difficulties in bearing defect diagnosis when the bearing has multiple defects increase, since unexpected changes occur in the amplitude of the bearing defect frequencies. In addition, the tendency toward condition-based maintenance (CBM) requires a better understanding of the fault progression due to the fact that multiple defects is one kind of fault development. In this paper, in order to detect multiple defects on one component of the bearing, a new method based on the high frequency resonance technique (HFRT) is introduced. The time constant in the envelope detector is used to find the pattern of the amplitude of defect frequency harmonics (ADFH) in the frequency domain. This method is based on a comparison of the ADFH with a curve, which is obtained from vibration modeling of the bearing. Two criteria are given for the diagnosis of multiple defects. The method is investigated with a simulation and a real experiment. Single and multiple defects are created on the outer race of the ball bearing at different angles. Additionally, the ADFH in the multiple faults experiments are calculated with the proposed mathematical modeling in order to check the accuracy of the model. The experimental results confirm the ability of the proposed method to diagnose multiple defects.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;135(1):011103-011103-11. doi:10.1115/1.4007759.

Experimental evidence in the literature suggests that foil bearing-supported rotors can suffer from subsynchronous vibration. While dry friction between top foil and bump foil is thought to provide structural damping, subsynchronous vibration is still an unresolved issue. The current paper aims to shed new light onto this matter and discusses the impact of various design variables on stable foil bearing-supported rotor operation. It is shown that, while a time domain integration of the equations of motion of the rotor coupled with the Reynolds equation for the fluid film is necessary to quantify the evolution of the rotor orbit, the underlying mechanism and the onset speed of instability can be predicted by coupling a reduced order foil bearing model with a rigid-body, linear, rotordynamic model. A sensitivity analysis suggests that structural damping has limited effect on stability. Further, it is shown that the location of the axial feed line of the top foil significantly influences the bearing load capacity and stability. The analysis indicates that the static fluid film pressure distribution governs rotordynamic stability. Therefore, selective shimming is introduced to tailor the unperturbed pressure distribution for improved stability. The required pattern is found via multiobjective optimization using the foil bearing-supported rotor model. A critical mass parameter is introduced as a measure for stability, and a criterion for whirl instability onset is proposed. It is shown that, with an optimally shimmed foil bearing, the critical mass parameter can be improved by more than two orders of magnitude. The optimum shim patterns are summarized for a variety of foil bearing geometries with different L/D ratios and different degrees of foil compliance in a first attempt to establish more general guidelines for stable foil bearing design. At low compressibility (Λ < 2), the optimum shim patterns vary little with bearing geometry; thus, a generalized shim pattern is proposed for low compressibility numbers.

Commentary by Dr. Valentin Fuster

Contact Mechanics

J. Tribol. 2012;135(1):011401-011401-10. doi:10.1115/1.4007760.

The elastic contact between two computer generated isotropic rough surfaces is studied. First the surface topography parameters including the asperity density, mean summit radius, and standard deviation of asperity heights of the equivalent rough surface are determined using an 8-nearest neighbor summit identification scheme. Second, many cross sections of the equivalent rough surface are traced and their individual topography parameters are determined from their corresponding spectral moments. The topography parameters are also obtained from the average spectral moments of all cross sections. The asperity density is found to be the main difference between the summit identification scheme and the spectral moments method. The contact parameters such as the number of contacting asperities, real area of contact, and contact load for any given separation between the equivalent rough surface and a rigid flat are calculated by summing the contributions of all the contacting asperities using the summit identification model. These contact parameters are also obtained with the Greenwood-Williamson (GW) model using the topography parameters from each individual cross section and from the average spectral moments of all cross sections. Three different surfaces and three different sampling intervals were used to study how the method to determine topography parameters affects the resulting contact parameters. The contact parameters are found to vary significantly based on the method used to determine the topography parameters, and as a function of the autocorrelation length of the surface, as well as the sampling interval. Using a summit identification model or the GW model based on topography parameters obtained from a summit identification scheme is perhaps the most reliable approach.

Commentary by Dr. Valentin Fuster

Elastohydrodynamic Lubrication

J. Tribol. 2012;135(1):011502-011502-10. doi:10.1115/1.4023082.

This paper investigates the effects of lubricant compressibility on the film-forming performance of thermal elastohydrodynamic lubricated (EHL) circular contacts. Numerical film thickness predictions using the classical Dowson and Higginson relationship are compared to those that would be obtained using a more realistic compressibility model, all other parameters kept unchanged. This allows an isolation of the realistic compressibility effects on the film-forming performance. For realistic predictions, the authors consider two model liquids from the 1953 report of the ASME Research Committee on Lubrication, the most and the least compressible. The compressibility of these liquids is modeled using the Tait equation of state (EoS) while all other transport properties are kept unchanged for the sake of isolating compressibility effects. In addition, the same typical generalized-Newtonian behavior is assumed for both model liquids. The results reconfirm the well-known observations that minimum film thickness is very little affected by lubricant compressibility while central film thickness decreases linearly with the increase in volume compression of the lubricant. It is also observed that the relative errors on central film thicknesses induced by the use of the Dowson and Higginson relationship for compressibility increase with load and temperature and are very little affected by mean entrainment speed. Compressibility is shown to be a significant source of error in film-derived measurements of pressure-viscosity coefficients especially at high temperature. The thermodynamic scaling that provides an accurate and consistent framework for the correlation of the thermophysical properties of liquids with temperature and pressure requires an accurate equation of state. In brief, this paper highlights the importance of using realistic transport properties modeling based on thermodynamic scaling for an accurate numerical prediction of the performance of EHL contacts.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;135(1):011503-011503-6. doi:10.1115/1.4007808.

The phenomena that occur when an elliptical steel body impacts a stationary steel plate with surface asperities are discussed through isothermal Newtonian numerical analysis using sinusoidal roughness. The ridges of the surface asperities produce large local pressures, especially at a large ellipticity ratio, when the surfaces are approaching each other under the applied load. The values of the local pressures are larger when the ridges are along the major axis than when the ridges are along the minor axis. Furthermore, as the loading speed increases, the pressure increases. As a result, the microgrooves are produced in the ridges and the horseshoe-shaped constrictions are formed at the ridges located around the contact edge.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;135(1):011504-011504-11. doi:10.1115/1.4007809.

A first-principle based mathematical model is developed in this paper to analyze the meshing losses in involute spur gears operating in high-load and high-speed conditions. The model is fundamentally simple with a few clearly defined physical parameters. It is computationally robust and produces meaningful trends and relative magnitudes of the meshing losses with respect to the variations of key gear and lubricant parameters. The model is evaluated with precision experimental data. It is then used to study the effects of various gear and lubricant parameters on the meshing losses including gear module, pressure angle, tooth addendum height, thermal conductivity, and lubricant pressure-viscosity and temperature-viscosity coefficients. The results and analysis suggest that gear module, pressure angle, and lubricant pressure-viscosity and temperature-viscosity coefficients can significantly affect the meshing losses. They should be the design parameters of interest to further improve the energy efficiency in high-performance, multistage transmission systems. Although the model is developed and results obtained for spur gears, the authors believe that the trends and relative magnitudes of the meshing losses with respect to the variations of the gear and lubricant parameters are still meaningful for helical gears.

Commentary by Dr. Valentin Fuster

Friction & Wear

J. Tribol. 2012;135(1):011602-011602-9. doi:10.1115/1.4023081.

During hot extrusion process, die wear shortens markedly the service life of extrusion dies under the high-pressure, high-temperature conditions. In this paper, based on modified Archard's wear model, a user-defined subroutine for calculating die wear depth was developed and implanted into DEFORM-3D. On the basis of the numerical model, the die wear behavior during aluminum alloy 7075 tube extrusion has been investigated. The numerical results show that process variables have multiple effects on die wear behavior. With the increasing ram speed, wear depth of die bearing rises and then tends to decline gradually. From the ram speed of 15 mm/s, die wear depth begins to increase again. Wear depth rises suddenly with the increase of friction coefficient, then gradually reduces. When friction coefficient is greater than 0.8, wear depth tends to be a constant. A maximum wear depth occurs at 430 °C of billet temperature, and a minimum wear depth occurs at certain die temperature in the range of 400–425 °C. In addition, the required extrusion force has strong dependence on process variables. The extrusion force rises clearly with the increase of ram speed and friction coefficient and with the decrease of initial temperatures of billet and die.

Commentary by Dr. Valentin Fuster

Hydrodynamic Lubrication

J. Tribol. 2012;135(1):011701-011701-12. doi:10.1115/1.4007884.

This paper presents theoretical models and simulation results for the synchronous, thermal transient, instability phenomenon known as the Morton effect. A transient analysis of the rotor supported by tilting pad journal bearing is performed to obtain the transient asymmetric temperature distribution of the journal by solving the variable viscosity Reynolds equation, a 2D energy equation, the heat conduction equation, and the equations of motion for the rotor. The asymmetric temperature causes the rotor to bow at the journal, inducing a mass imbalance of overhung components such as impellers, which changes the synchronous vibrations and the journal's asymmetric temperature. Modeling and simulation of the cyclic amplitude, synchronous vibration due to the Morton effect for tilting pad bearing supported machinery is the subject of this paper. The tilting pad bearing model is general and nonlinear, and thermal modes and staggered integration approaches are utilized in order to reduce computation time. The simulation results indicate that the temperature of the journal varies sinusoidally along the circumferential direction and linearly across the diameter. The vibration amplitude is demonstrated to vary slowly with time due to the transient asymmetric heating of the shaft. The approach's novelty is the determination of the large motion, cyclic synchronous amplitude behavior shown by experimental results in the literature, unlike other approaches that treat the phenomenon as a linear instability. The approach is benchmarked against the experiment of de Jongh.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;135(1):011702-011702-13. doi:10.1115/1.4007885.

This work presents an optimization procedure to find bearing profiles that improve stability margins of rotor-bearing systems. The profile is defined by control points and cubic splines. Stability margins are estimated using bearing dynamic coefficients, and obtained solutions are analyzed as a function of the number of control points and of the Sommerfeld number at optimization. Results show the feasibility of finding shapes for the bearing that significantly improve the stability margins. Some of the obtained solutions overcome the stability margins of conventional bearings, such as the journal bearing and preloaded bearings with 0.5 and 0.67 preload. A time domain simulation of a flexible shaft rotating system supported by such bearings corroborates the results.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Tribol. 2012;135(1):014501-014501-5. doi:10.1115/1.4023080.

Research on the applications of magneto-rheological (MR) elastomers in mechanical engineering has greatly expanded, whereas the performance of MR fluids in tribology has rarely been investigated. In this study, the tribological characteristics of an MR elastomer are identified in order to improve tribological performance with the activation of a magnetic field. Microscopic changes in the surface and in the MR particles are investigated. The friction and wear of an MR elastomer is measured using a pin-on-disc tester under applied and unapplied magnetic fields. In addition, the linear sliding friction of an MR elastomer with respect to different velocities and loads is measured using a linear sliding tester.

Commentary by Dr. Valentin Fuster
J. Tribol. 2012;135(1):014502-014502-7. doi:10.1115/1.4007575.

This paper deals with the analysis of eroded surfaces obtained from cavitation-erosion experiments on stainless steel in water and oil-in-water (o/w) emulsions using image processing. Two analysis techniques that are very promising in this respect are the wavelet decomposition transform and fractals. These can be used to extract parameters that characterize the cavitation intensity in a similar manner to that of the mean depth of erosion (MDP). The extracted parameters are the wavelet energy and entropy as well as the fractal dimension. Both of the image feature parameters and the MDP decrease with adding oil to water. Also, it was found that the variation of image feature parameters versus concentration of oil-in-water emulsions has a general trend that does not depend on magnification factor. The cavitation erosion behavior and mechanism for water and o/w emulsions were analyzed and it was found that the predominant failure mode was fatigue for water and o/w emulsions. The results show that a corrosive effect appears at 5 and 10 wt% o/w emulsions.

Commentary by Dr. Valentin Fuster

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