Research Papers: Applications

J. Tribol. 2014;136(3):031101-031101-10. doi:10.1115/1.4027130.

This article deals with the effects of the contact ratio ε on transmission errors of trochoidal gears (which consist of a roller gear anda cam gear). First, the experiments and multibody analysis (MBA) for the transmission errors of two types of single-row trochoidal gears (types A and B gears) were carried out. The type A gear is a commercial trochoidal gear with ε = 1.1 and the type B gear is a trochoidal gear with ε = 2.1 (by increasing the number of teeth). The experimental and MBA results showed that the peak-to-peak value TEP-P of the transmission errors of the type B gear (with ε = 2.1) was lower than the type A gear (with ε = 1.1). The TEP-P of types A and B gears increased as the rotational speed of the roller gear increased. However, the increasing rate of the measured TEP-P of the type B gear due to an increase of the rotational speed was less than that of the type A gear. Increasing the contact ratio due to an increase in the number of teeth in a single-row trochoidal gear (such as a type B gear) decreases the strength of the teeth and rollers. To overcome this problem, as a new transmission error reduction method, a double-row trochoidal gear (type C gear), having two times the contact ratio of the type A single-row trochoidal gear was presented and its transmission error was examined. The experimental and MBA results showed that the TEP-P of the transmission errors of the type C double-row trochoidal gear were lower than that of the type A single-row trochoidal gear. Therefore, it is clear that using a double-row trochoidal gear is effective for reducing the transmission errors of trochoidal gears.

Topics: Gears , Errors
Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031102-031102-13. doi:10.1115/1.4027438.

Minimizing the clearance between turbofan blades and the surrounding casing is a key factor to achieving compressor efficiency. The deposition of an abradable coating on casings is one of the technologies used to reduce this blade-casing clearance and ensure blade integrity in the event of blade-casing contact. Aircraft in-service conditions may lead to interactions between the blade tip and the coated casing, during which wear of the abradable coating, blade dynamics, and interacting force are critical yet little-understood issues. In order to study blade/abradable-coating interactions of a few tens of milliseconds, experiments were conducted on a dedicated test rig. The experimental data were analyzed with the aim of determining the friction-induced vibrational modes of the blade. This involved a time-frequency analysis of the experimental blade strain using continuous wavelet transform (CWT) combined with a modal analysis of the blade. The latter was carried out with two kinds of kinematic boundary conditions at the blade tip: free and modified, by imposing contact with the abradable coating. The interaction data show that the blade vibration modes identified during interactions correspond to the free boundary condition due to the transitional nature of the phenomena and the very short duration of contacts. The properties of the continuous wavelet transform were then used to identify the occurrence of blade-coating contact. Two kinds of blade/abradable-coating interactions were identified: bouncing of the blade over short time periods associated with loss of abradable material and isolated contacts capable of amplifying the blade vibrations without causing significant wear of the abradable coating. The results obtained were corroborated by high-speed imaging of the interactions.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2014;136(3):031401-031401-8. doi:10.1115/1.4027544.

Our investigations aimed to model the thermal stress development between wheel and rail, caused by heat generation during braking, by coupled transient thermal and elastic-plastic FE simulations. Stresses are generated due to thermal expansion caused by local temperature rise and changes in temperature in case of one revolution of the wheel. Our investigations resulted in the fact that thermal expansion caused by heat generation and heat conduction induced considerable local stresses along the thread of the wheel ∼0.1–0.5 mm underneath the surface.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031402-031402-10. doi:10.1115/1.4027240.

The macromechanical tribological mechanism describes the friction phenomenon by considering the stress and the strain distributions, and the total elastic and plastic deformations. Based on the finite element method (FEM), the elastoplastic frictional contact problem is formulated as an incremental convex programming model (CPM). The Lagrange multiplier approach is adopted for imposing the inequality contact constraints. The Coulomb's friction law and the Prandtl–Reuss flow rule are used for the friction conditions and the elastoplastic behavior, respectively. The frictional contact examples are analyzed using the developed adaptive incremental procedure to elucidate the tribological behavior of the contact bodies and the model applicability.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2014;136(3):031501-031501-8. doi:10.1115/1.4027286.

The effects of nanotexturing on oil film thickness and shape under pointcontact elasto-hydrodynamic lubrication (EHL) conditions were experimentally investigated. A disk-onball friction tester with an optical interferometer was used to measure oil film thickness and to observe the oil film shape. Periodic groove structures with a spiral, perpendicular, or parallel shape and with various groove depths and distances were formed by irradiation of a femtosecond laser onto the surface of steel balls. These nanotextured balls were tested under a load of 20 N and at rotational speeds from 1.0 to 3.0 m/s. Most of the balls with nanotexturing had a thicker oil film than those without texturing. The groove depth and angle were the key parameters determining the thickness of the oil film as they controlled the amount of side leakage of oil from the contact point. Optimization of these parameters resulted in an oil film that was almost twice as thick as that on the ball without texturing.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031502-031502-9. doi:10.1115/1.4027210.

The effects of the transversely isotropic coating layer on the elastohydrodynamic lubrication (EHL) circular contact problems are analyzed and discussed under constant load condition. The equivalent elastic modulus for an equivalent isotropic half-space problem is applied to simplify the present transversely isotropic coating. The finite element method (FEM) is utilized to solve the Reynolds equation, the load balance equation, the rheology equations, and the elastic deformation equation simultaneously. The simulation results of the present equivalent model are compared with those of an anisotropic material elasticity matrix to evaluate the applicable range of coating thickness under a fixed relative error. The pressure distribution tends to gradually escalating and concentrating toward the center with increasing longitudinal Young's modulus. The variations of pressure and film thickness become significant as the coating thickness becomes thinner. The deformations of interface are smaller than the deformations of the surface. The film thickness and pressure characteristics of the lubricant are discussed for various parameters. These characteristics are important for the design of the mechanical element with coating layer.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031503-031503-7. doi:10.1115/1.4027480.

The numerical studies on the influences of surface parameters skewness and kurtosis on tribological characteristics under mixed elastohydrodynamic lubrication (mixed EHL) conditions are extended to fatigue life. Non-Gaussian rough surfaces are generated numerically with given autocorrelation function, skewness, and kurtosis. The results show that the maximum pressure increases as the skewness increases, however its variation with kurtosis is closely related to skewness. Similar trends to that of the maximum pressure are observed for the maximum von Mises stress. The fatigue life decreases as the skewness increases, however it undergoes apparent fluctuations with the increase of kurtosis. As the kurtosis increases, the influence of skewness on fatigue life becomes more significant, and vice versa.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031504-031504-11. doi:10.1115/1.4027478.

Efficiency and durability are among the top concerns in mechanical design to minimize environmental impact and conserve natural resources while fulfilling performance requirements. Today mechanical systems are more compact, lightweight, and transmit more power than ever before, which imposes great challenges to designers. Under the circumstances, some simplified analyses may no longer be satisfactory, and in-depth studies on mixed lubrication characteristics, taking into account the effects of 3D surface roughness and possible plastic deformation, are certainly needed. In this paper, the recently developed plasto-elastohydrodynamic lubrication (PEHL) model is employed, and numerous cases with both sinusoidal waviness and real machined roughness are analyzed. It is observed that plastic deformation may occur due to localized high pressure peaks caused by the rough surface asperity contacts, even though the external load is still considerably below the critical load determined at the onset of plastic deformation in the corresponding smooth surface contact. It is also found, based on a series of cases analyzed, that the roughness height, wavelength, material hardening property, and operating conditions may all have significant influences on the PEHL performance, subsurface von Mises stress field, residual stresses, and plastic strains. Generally, the presence of plastic deformation may significantly reduce some of the pressure spikes and peak values of subsurface stresses and make the load support more evenly distributed among all the rough surface asperities in contact.

Commentary by Dr. Valentin Fuster

Research Papers: Friction & Wear

J. Tribol. 2014;136(3):031601-031601-6. doi:10.1115/1.4026504.

Carbon nanotubes (CNTs) coated on a tungsten carbide/cobalt (WC/Co) micropunch (150 μm in diameter) and their effect on the wear of micropunches were investigated. CNTs were synthesized by homemade method. After the punching test with Ti as the substrate, the effect of CNTs on the wear loss and the surface morphology of the micropunch had been studied by confocal laser, scanning electron microscopy (SEM), and digital balance. Results show the wear of a CNTs coated micropunch obviously decreases. Even in the severe wear period, the wear loss is less than that of a non-CNTs coated micropunch. Compared with the micropunch without CNTs coating, the promising results are due to the formation of a lubrication film at the contact region by rubbing of the CNT forest; CNTs produced adhere to the micropunch surface avoiding direct contact during the punching period and providing lubricant properties to the interface by virtue of their graphitic nature.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031602-031602-7. doi:10.1115/1.4027440.

In this study, the wear depths under different loads, speeds, lubricant temperatures, and surface roughness amplitudes are experimentally determined using a twin-disk rolling contact setup. A point contact wear model combining a contact formulation and Archard's wear equation in an iterative manner is developed to simulate the wear process of the experiments. By matching the measured and predicted wear profiles, the wear coefficients under different operating and surface conditions are determined. It is found that the wear coefficient increases when either the load or the surface roughness amplitude increases and decreases as the lubricant pressure-viscosity coefficient increases. Within the operating ranges considered, it is observed that the lubricant pressure-viscosity coefficient is the most influential parameter on wear, the load has the least impact, and the surface roughness amplitude is in between. Lastly, a regression formula is given for the estimation of Archard's wear coefficient.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031603-031603-9. doi:10.1115/1.4027479.

A top-down approach is employed to investigate the tribological effect of adding nanographite platelets (NGPs) to mineral base oil (MBO). The performance of the NGP-modified MBO was evaluated by examining the friction and anti-wear properties. Four different types of NGPs produced by two different processes were employed. The optimal NGP-modified MBO attained a significant wear and friction reduction when compared with the MBO without NGPs. The process used to exfoliate the graphite nanoplatelet samples provided better wear properties because of the graphene layers' smoother sliding mechanism. Graphene layers seeped inside the groove marks to keep the friction coefficient low.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031703-031703-13. doi:10.1115/1.4026972.

Nature has long been an important source of inspiration for mankind to develop artificial ways to mimic the remarkable properties of biological systems. In this work, a new method was explored to fabricate a biomimetic engineering surface comprising both the shark-skin, the shark body denticle, and rib morphology. It can help reduce water resistance and the friction contact area as well as accommodate lubricant. The lubrication theory model was established to predict the effect of geometric parameters of a biomimetic surface on tribological performance. The model has been proved to be feasible to predict tribological performance by the experimental results. The model was then used to investigate the effect of the grid textured surface on frictional performance of different geometries. The investigation was aimed at providing a rule for deriving the design parameters of a biomimetic surface with good lubrication characteristics. Results suggest that: (i) the increase in depression width ratio Λ decreases its corresponding coefficient of friction, and (ii) the small coefficient of friction is achievable when Λ is beyond 0.45. Superposition of depth ratio Γ and angle's couple under the condition of Λ < 0.45 affects the value of friction coefficient. It shows the decrease in angle decreases with the increase in dimension depth Γ.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2014;136(3):031701-031701-10. doi:10.1115/1.4026887.

An aerostatic bearing has been designed for supporting a heavy payload. The bearing involves an axial gap on the stator top that provides an upward lift, a bottom gap for counterbalancing the tendency of large lift-off, and a feeding orifice at the bearing inlet that is connected with the gaps by a network of holes or inherences yielding the damping. Notable contributions of the work are proving the concept by numerical simulation through first-principle order-separated modeling and evolving a simple solution strategy. The predicted vertical motion dynamics of the payload reveals that, depending on the target range of the payload weight, alternatives could be free or choked orifice designs.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031702-031702-4. doi:10.1115/1.4026592.

While at high pressure, the classical Navier–Stokes equation is suitable for modeling squeeze-film damping, at low pressure, it needs some modification in order to consider fluid rarefaction. According to a common approach, fluid rarefaction can be included in this equation by substituting the standard fluid viscosity with a fictitious quantity, known as effective viscosity, for which different formulations were proposed. In order to identify which expression works better, the results obtained when either formulation is implemented inside the Navier–Stokes equation (that is then solved by both analytical and numerical means) are compared with already available experimental data. At the end, a novel expression is discussed, derived from a computer-assessed optimization procedure.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031704-031704-9. doi:10.1115/1.4026590.

Annular seals are used in turbomachinery to reduce secondary flow between regions of high and low pressure. In a vibrating rotor system, the nonaxisymmetric pressure field developed in the small clearance between the rotor and the seal generate reactionary forces that can affect the stability of the entire rotor system. Traditionally, two analyses have been used to study the fluid flow in seals, bulk-flow analysis and computational fluid dynamics (CFD). Bulk-flow methods are computational inexpensive, but solve simplified equations that rely on empirically derived friction factor coefficients and are moderately accurate. CFD analyses generally provide more accurate results than bulk-flow codes, but solution time can vary between days and weeks. For gas damper seals, these analyses have been developed with the assumption that the flow can be treated as isothermal. However, some experimental studies have shown that the temperature change across the seal can be as much as 37%. Thus, a comprehensive analysis requires the solution of an energy equation. Recently, a new hybrid method that employs a CFD analysis for the zeroth-order flow and a bulk-flow analysis for the first-order, perturbed flow has been developed. This method has shown to compare well with full CFD analysis and experimental data while being computationally efficient. In this study, the previously developed hybrid method is extended to include the effects of nonisothermal flow. The hybrid method with energy equation is then compared with the isothermal hybrid method and experimental data for several test cases of hole-pattern seals and the importance of the use of energy equation is studied.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031705-031705-7. doi:10.1115/1.4027293.

According to the extended Reynolds theory, surface roughness contributes to the pressure buildup as well as shear stress and transport in the film flow. The effect is usually quantified using pressure and shear flow factors. The influence of the pattern directionality relative to the sliding motion may be considered using an anisotropic model of flow factors. The goal of the present study is to quantify these effects based on a precise numerical solution of the Navier–Stokes equations. For the computation the open source finite volume code OpenFOAM is used. The computational setup allows consideration of the lubrication film between two rough surfaces in relative motion. The roughness of the surfaces is simplified and parameterized using trigonometric functions.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031706-031706-14. doi:10.1115/1.4027309.

The present study is focused on accurate prediction of the Morton effect problem including journal asymmetric heating and the corresponding long period amplitude oscillations using a nonlinear time transient rotor-dynamic simulation. This paper presents a theoretical model of thermal induced synchronous instability problems in a nonlinear rotor–bearing system, and suggests a new computational algorithm for the nonlinear transient analysis of the Morton effect where the dynamic and thermal problems are combined. For the analysis of the Morton effect problem, a variable viscosity Reynolds equation and a 3D energy equation are coupled via temperature and viscosity, and solved simultaneously. Three-dimensional heat transfer equations of bearing and shaft are modeled by a finite element method, and thermally coupled with the fluid film via a heat flux boundary condition. Asymmetric heat flux into the synchronously whirling rotor is solved by the orbit time averaged heat flux from fluid film to the spinning shaft surface. The journal orbit is calculated by the nonlinear transient dynamic analysis of a rotor–bearing system with a variable time step numerical integration scheme. For the computation time reduction, modal coordinate transformation is adopted in dynamic and thermal transient analysis. Thermal bow effect makes a significant change to the dynamic behavior of a rotor–bearing system, and a thermal hysteresis bode plot, that is one of the characteristics of the Morton effect problem, is presented with time varying spin speed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031707-031707-16. doi:10.1115/1.4027310.

This paper presents simulation results corresponding with the theoretical Morton effect model explained in Part I, where the 3D finite element models of bearing, shaft, and fluid film are adopted. In addition, it explains how thermal bow induced imbalance force develops in the spinning journal with time and how the vibration level is affected by the thermal bow vector. Shaft asymmetric thermal expansion induced by nonuniform journal heating is simulated, which is one of the unique contributions of this research. The effect of changes in: (1) thermal boundary condition around the pad, (2) lubricant supply temperature, (3) initial mechanical imbalance, (4) pivot stiffness, (5) film clearance, and (6) pad material are studied. Cooling the pad and the lubricant, using a pad with a low thermal expansion coefficient, soft pivot, and reducing the initial imbalance are found to be the best remedies for the thermal induced synchronous rotor instability problem.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031708-031708-5. doi:10.1115/1.4027398.

The current experimental researches on the orbit of a journal center of a crankshaft bearing for an internal combustion engine were usually focused on the 2D movement locus of a crankshaft journal center in the cross section of the bearing. However, in the actual operation of an internal combustion engine, there exists the movement of a crankshaft journal along the bearing axis under the effect of various factors, such as the crankshaft deformation acted by load. Obviously the tribological performance of a crankshaft bearing is affected inevitably by the movement of the crankshaft journal along the bearing axis. In this paper, a four-stroke four-cylinder internal combustion engine was taken as the studying object, the 3D orbit (that includes the movement in the cross section of the bearing and the movement along the bearing axis) of the journal center of the crankshaft bearing for an internal combustion engine was measured under a number of operating conditions on the test bench of an internal combustion engine. The position of the journal in the crankshaft bearing was obtained by the measurement using eddy current gap sensors and the data post-process. The results show that there exists the movement of the crankshaft journal along the axial direction in the bearing for an internal combustion engine. The actual orbit of the journal center of the crankshaft bearing for an internal combustion engine is a 3D spatial curve. The orbit of the journal center of the crankshaft bearing in one operating cycle of an internal combustion engine is not a closed curve. There is relatively a large movement of the journal along the axial direction of the crankshaft bearing, and the numerical value of the movement is greater than the radial clearance of the bearing. The greater the rotational speed of the internal combustion engine, the larger the amount of axial movement of the journal. The periodic variation exists in the axial movement of the bearing journal in one operating cycle of the internal combustion engine at low engine speed, and the varying periodicity equals the number of engine cylinders. There is no obvious varying rule of the axial movement of the bearing journal in one operating cycle of the internal combustion engine at high engine speed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031709-031709-6. doi:10.1115/1.4027399.

This paper presents an optimization algorithm for journal bearing profiles with respect to stiffness and load-carrying capacity and an application example of the same. An analytical approach for finding the optimum is derived. How the optimization procedure works when using numerical calculation tools is briefly explained. The application example is introduced and the expected performance of the optimized bearing profile is compared to the expected performance of state-of-the-art bearing profiles. Finally, the measured bearing performance data is compared to the expected values. The findings indicate the effectiveness of the introduced optimization algorithm.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):031710-031710-10. doi:10.1115/1.4027548.

A tribo-dynamic model of a spiral-groove rotary seal ring is developed through coupling lubrication and dynamic equations. Effects of centrifugation, hydrodynamics, cavitation, and asperity contact are considered. To represent real rough surfaces, asperity contact is described by a statistics-based model. A global time marching scheme is developed to obtain the motion of seal ring and key parameters such as bearing force, friction torque, and leakage rate. Dynamic behaviors and seal characteristics of spiral-groove rotary seal ring under real and step change oil filling conditions are analyzed. The result shows that the rotary seal ring operates steadily under real conditions and has fast and stable step response. It is also indicated that the seal ring can achieve full film lubrication under high speed conditions through the oil filling and dispersing stage. The steady lubrication performance is experimentally validated.

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2014;136(3):031801-031801-6. doi:10.1115/1.4027543.

Different blends containing 5%, 10%, 15%, 20%, 25%, and 30% of polyalphaolefin-100 (PAO-100) as a minor component and solvent neutral-150 (SN-150) as a major component were prepared. The important lubrication properties of the blends were then measured. The obtained results indicate that upon increasing the weight percentage of PAO-100, the viscosity index (VI) rises, the cold cranking simulator (CCS) diminishes, and viscosities approach the corresponding values due to solvent neutral-500 (SN-500). Different equations were derived between the viscosity and VI and weight percentage of PAO-100. A similar procedure was employed for blends containing 1%, 5%, 10%, 12%, and 15% of polyisobutene (PIB). In this case, results parallel to PAO-100 were obtained. The 30% PAO-100: SN-150 and 12% PIB: SN-150 mixtures were successfully employed for the equalization of semisynthetic engine oil SAE 20W50 API SJ from Millers Company. Biodegradability as well as tribotests were performed on the recent blends and compared with corresponding values due to SN-150.

Commentary by Dr. Valentin Fuster

Research Papers: Magnetic Storage

J. Tribol. 2014;136(3):031901-031901-5. doi:10.1115/1.4027209.

The work performance of a hard disk drive (HDD) in mobile devices depends very much on its ability to withstand external disturbances. In this study, a detailed multibody structural model integrated with a complete air bearing model is developed to investigate the disk drive's response during external shocks. The head disk interface (HDI) failure mechanisms when the HDD is subjected to different shock cases are discussed. For a negative shock case in which the disk initially moves towards the head, with long pulse width, the air bearing becomes very stiff before the slider crashes on the disk, and the HDI fails only when the external load overcomes the air bearing force. For other shock cases, the slider contacts the disk due to a negative net bearing force caused by the slider-disk separation. Finally, a stiffer suspension design is proposed to improve the drive shock performance, especially during a positive shock, as under these conditions, the slider contacts the disk primarily due to the stiffness difference of the different drive components.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Tribol. 2014;136(3):034501-034501-5. doi:10.1115/1.4026503.

Nanoparticles-laden gas film (NLGF) was formed by adding SiO2 nanoparticles with volume fraction in the range of 0.014–0.330% and size of 30 nm into the air gas film in a thrust bearing. An effective viscosity of the gas-solid two phase lubrication media was introduced. The pressure distribution in NLGF and the load capacity of the thrust bearing were calculated by using the gas-solid two phase flow model with the effective viscosity under the film thicknesses range of 15–60 μm condition. The results showed that the NLGF can increase the load capacity when the film thickness is larger than 30 μm. The mechanism of the enhancement effect of load capacity was attributed to the increase of the effective viscosity of the NLGF from the pure air film, and the novel lubrication media of the NLGF can be expected for the bearing industry application.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):034502-034502-4. doi:10.1115/1.4026886.

In this paper, three lubricants are tested by a four-ball tribo-tester and in a cold rolling process. Our experimental results show that the data from the four-ball tests and the cold rolling tests are generally consistent with each other, implying the tribological properties measured by a four-ball tribo-tester are able to indicate the lubricants' lubricating performance in cold rolling reasonably well. Nevertheless, our experimental results also reveal certain discrepancies between the data from the four-ball tests and cold rolling tests. Especially, the friction coefficients from the four-ball tests are always considerably smaller than their counterparts from the cold rolling tests. Based on our analysis, the friction coefficients measured by cold rolling tests appear to be a more reliable indicator for the lubricants' performance in cold rolling than the friction coefficients measured by the four-ball tests.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):034503-034503-5. doi:10.1115/1.4027132.

We have used an ultrasonic method to determine the normal and shear stiffness for three different surfaces. The degree of hysteresis for the loading/unloading and stiffness ratio is a function of roughness. Nonlinear contact stiffness characteristics are obtained. The ratio of tangential to normal stiffness KT/KN slowly increases in proportion to normal loading. The novelty of our setup is that at the same time we can measure the reflection coefficient, obtain results for three transducers simultaneously, and measure the approach as a function of load. The presented experimental results of normal contact stiffness measurements have been used for the verification of our theoretical model based on a fractal description of rough surfaces (Buczkowski et al., “Fractal Normal Contact Stiffness of Rough Surfaces,” Arch. Mech. (submitted for publication).

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):034504-034504-6. doi:10.1115/1.4027388.

The heat generated during the sliding period at the initiation of engagement in friction clutches is considered to be one of the main reasons for the failure of the friction material. One way to reduce the risk of this problem is to increase the rate of heat transfer by convection or, in other words, reduce the heat content of the friction material (internal energy) and thereby increase the lifecycle of the friction clutch. In this paper, the finite element technique has been used to study the effect of radial circumferential grooves on the temperature distribution and the amount of energy transferred by convection for a dry friction clutch disk during a single engagement, assuming a uniform distribution for the thermal load between the contact surfaces (i.e., uniform wear on clutch surfaces). Three-dimensional transient simulations are conducted to study the thermoelastic coupling of the problem. The effect of the groove area ratio (GR, defined as the groove area divided by the nominal contact area) is investigated. Furthermore, this paper presents the equations for energy considerations and energy balance at any time for the friction clutch system. The numerical results show that the amount of energy transferred by convection from the friction material can be controlled (within a limitation) by adjusting the value of the groove area ratio. Commercial ANSYS13 software has been used to perform the numerical computations in this paper.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(3):034505-034505-6. doi:10.1115/1.4027400.

The nanoparticles-laden gas film (NLGF), which is formed by adding nanoparticles into the gas film, has a potential to increase the load capacity of the gas film and to protect the surfaces of the bearing from severe contact damage. In order to explore the lubrication performance of NLGF, the load capacity in the noncontact state and the friction coefficient in the contact state were studied experimentally by a novel NLGF thrust bearing apparatus. The effects of nanoparticles concentration on the load capacity and the friction coefficient were investigated, respectively. The lubrication performance of NLGF in a 200 start-stop cyclic test was evaluated. The contact surfaces were analyzed by the surface profilometer, scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). The results showed that NLGF had the enhancement of the load capacity in the noncontact state and possessed the properties of friction reduction and surface protection in the contact state. An optimal nanoparticles concentration of 60 g/m3 was found, making NLGF have a relative high load capacity in the noncontact state and the lowest friction coefficient in the contact state. With the optimal concentration, the friction coefficient with NLGF kept a low value during the 200 start-stop cyclic test. Then the friction reduction mechanism of NLGF was discussed, and it was inferred that the surface of the disk was covered with a protective film formed by nanoparticles, leading to a lower shear force. This study opens new perspectives of adding nanoparticles into gas bearings to improve the lubrication performance.

Commentary by Dr. Valentin Fuster

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